Why does anything exist ... why not nothing?

He is the image of the invisible God , the firstborn of all creation: for by Him all things were created, both in the heavens and on earth, visible and invisible, whether thrones, or dominions, or rulers, or authorities—all things have been created through Him and for Him. He is before all things, and in Him all things hold together. He is also the head of the body, the church; and He is the beginning, the firstborn from the dead, so that He Himself will come to have first place in everything. For it was the Father’s good pleasure for all the fullness to dwell in Him. - Colossians 1:15

Epicureanism

Epicurus
Epicureanism, founded by Epicurus (341–270 BCE), holds that the world is composed of atoms and void, and that natural phenomena should be understood through natural causes rather than divine intervention. It teaches that the purpose of life is to attain happiness and tranquility through the pursuit of pleasure, understood not as indulgence in luxury or sensuality, but as the absence of pain and disturbance.
Its basic philosophical structure can be simplified into four ideas:

Nature: Reality is material and operates according to natural processes. The gods, if they exist, do not intervene in human affairs.

Knowledge: We should rely on experience, sensation, and reason rather than fear, superstition, or unsupported speculation.

Human flourishing: The good life is primarily one of tranquility, freedom from unnecessary fear, and modest pleasure, rather than wealth, status, or excess.

Death: Death is not something to fear because, when we exist, death is not present; when death is present, we no longer exist.

Epicureanism therefore combines a natural philosophy with an ethical philosophy:

Understand nature correctly → remove false fears → live simply → achieve tranquility.
Misconceptions
In modern popular usage, an Epicurean is a connoisseur of the arts of life and the refinements of sensual pleasures; Epicureanism implies a love or knowledgeable enjoyment especially of good food and drink.
Because Epicureanism posits that pleasure is the ultimate good (telos), it has been commonly misunderstood since ancient times as a doctrine that advocates the partaking in fleeting pleasures such as sexual excess and decadent food. This is not the case. Epicurus regarded ataraxia (tranquility, freedom from fear) and aponia (absence of pain) as the height of happiness. He also considered prudence an important virtue and perceived excess and overindulgence to be contrary to the attainment of ataraxia and aponia.

What it is not!
Stoicism
Opposite to
the Stoics, where logic was the part of philosophy which examined reason. To achieve a happy life—a life worth living—requires logical thought
.
Also it is not Hedonism.
A philosophical theory that suggests pleasure is the ultimate good and the primary driver of human action. It proposes that actions are morally right if they lead to pleasure or happiness and wrong if they cause pain or suffering. Hedonists strive to maximize pleasure and minimize pain in their lives.
Hedonism’s basic tenets:
Pleasure as the Highest Good: Hedonism posits that pleasure is the only intrinsic good, meaning it is inherently good and valuable in itself.
Motivation: Individuals are inherently motivated to seek pleasure and avoid pain.
Moral Framework: Actions are judged based on their ability to produce pleasure or reduce pain.
Epicureanism stood in contrast to other schools like Stoicism, which emphasized duty and virtue over pleasure. Though often misunderstood as promoting hedonism, true Epicureanism advocates for a life of reasoned choice, modest living, and emotional serenity.

What is it?
Few of Epicurus’ writings survive, and those that do reflect his interest in applying Democritus’ theories to assist people in taking responsibility for themselves and for their own happiness—since he held there are no gods around that can help them.
Epicurus believed that the greatest good was to seek modest, sustainable pleasure in the form of a state of ataraxia (tranquility and freedom from fear) and aponia (the absence of bodily pain) through knowledge of the workings of the world and limiting desires.
Epicureans thought that sensations could not deceive, sensations are the first and main criterion of truth for Epicureans.
Our feelings or emotions (pathê) are how we perceive pleasure and pain. They are analogous to sensations in that they are a means of perception, but they perceive our internal state as opposed to external things
Epicureanism bases its ethics in such a way as to derive the greatest amount of pleasure possible during one’s lifetime, yet doing so moderately in order to avoid the suffering incurred by overindulgence in such pleasure.
When we say … that pleasure is the end and aim, we do not mean the pleasures of the prodigal or the pleasures of sensuality, as we are understood to do by some through ignorance, prejudice or willful misrepresentation. By pleasure we mean the absence of pain in the body and of trouble in the soul. It is not by an unbroken succession of drinking bouts and of revelry, not by sexual lust, nor the enjoyment of fish and other delicacies of a luxurious table, which produce a pleasant life; it is sober reasoning, searching out the grounds of every choice and avoidance, and banishing those beliefs through which the greatest tumults take possession of the soul. —Epicurus, “Letter to Menoeceus”
Emphasis was placed on pleasures of the mind rather than on physical pleasures. The Epicureans further divided each of these types of pleasures into two categories: kinetic pleasure and katastematic pleasure. Absence of pain, aponia, and lack of disturbance of mind, ataraxia, are two of the katastematic pleasures and often seen as the focal ones to Epicurus.
Kinetic pleasure is the physical or mental pleasures that involve action or change. Eating delicious food, as well as fulfilling desires and removing pain, which is itself considered a pleasurable act, are all examples of kinetic pleasure in the physical sense. According to Epicurus, feelings of joy would be an example of mental kinetic pleasure.
Katastematic pleasure is the pleasure one feels while in a state without pain – tranquility. Like kinetic pleasures, katastematic pleasures can also be physical, such as the state of not being thirsty, or mental, such as freedom from a state of fear.

Desire
Epicurus distinguished between different types of desires. Natural and necessary desires, such as hunger and thirst, are easy to satisfy and lead to minimal pain. Unnecessary desires, like wealth and fame, are more difficult to satisfy and can lead to greater suffering.
I learn that your bodily inclination leans most keenly towards sexual intercourse. If you neither violate the laws nor disturb well established morals nor sadden someone close to you, nor strain your body, nor spend what is needed for necessities, use your own choice as you wish. It is sure difficult to imagine, however, that none of these would be a part of sex because sex never benefited anyone. — Epicurus, Vatican Sayings, LI
In order to do this an Epicurean had to control their desires, because desire itself was seen as painful. Not only will controlling one’s desires bring about aponia, as one will rarely suffer from not being physically satisfied, but controlling one’s desires will also help to bring about ataraxia because one will not be anxious about becoming discomforted since one would have so few desires anyway.

The Epicureans divide desires into three classes: natural and necessary, natural but not necessary, and vain and empty.
Natural and necessary: These desires are limited desires that are innately present in all humans; it is part of human nature to have them. They are necessary for one of three reasons:
 necessary for happiness
 necessary for freedom from bodily discomfort
 and necessary for life.
Clothing and shelter would belong to the first two categories, while something like food would belong to the third.
Natural but not necessary: These desires are innate to humans, but they do not need to be fulfilled for their happiness or their survival.
Wanting to eat delicious food when one is hungry is an example of a natural but not necessary desire.
The main problem with these desires is that they fail to substantially increase a person’s happiness, and at the same time require effort to obtain and are desired by people due to false beliefs that they are actually necessary.
Vain and empty: These desires are neither innate to humans nor required for happiness or health; indeed, they are also limitless and can never be fulfilled. Desires of wealth or fame would fall in this class, and such desires are to be avoided because they will ultimately only bring about discomfort.
If one follows only natural and necessary desires, then, according to Epicurus, one would be able to reach aponia and ataraxia and thereby the highest form of happiness. Unnecessary and, especially, artificially produced desires were to be suppressed.
Epicureanism also resembles Buddhism in its belief that great excess leads to great dissatisfaction.

Key Tenets of Epicureanism:
Pleasure as the Highest Good:
The ultimate goal of life is pleasure, defined as freedom from bodily pain (aponia) and mental distress (ataraxia).
Simple pleasures, like friendship, knowledge, and moderation, are the most satisfying.
Materialism and Atomism:
Matter is the fundamental substance in nature, and all things, including mental states and consciousness, are results of material interactions. According to philosophical materialism, mind and consciousness are caused by physical processes, such as the neurochemistry of the human brain and nervous system, without which they cannot exist. Materialism directly contrasts with monistic idealism, according to which consciousness is the fundamental substance of nature.
Epicurus adopted Democritus’s atomic theory, believing that everything, including the soul, is made of atoms and void.
There is no afterlife; death is simply the end of consciousness and thus not to be feared.
Ethics of Moderation:
Not all pleasures should be pursued; some desires (like natural and necessary ones) should be satisfied, while others (vain or unnatural) should be avoided.
Mental pleasures are superior to physical ones.
Avoidance of Fear:
Philosophy helps eliminate fears—especially fear of gods and death—leading to inner peace.
The gods exist but are indifferent to human affairs, so they are not to be feared.
Self-sufficiency and Friendship:
Living a simple, self-sufficient life in the company of good friends is ideal.
Community and emotional support are essential to achieving a tranquil life.
In essence, Epicureanism is a philosophy that encourages a life of moderation, simplicity, and intellectual engagement, aiming for a state of lasting happiness through the avoidance of pain and the cultivation of meaningful relationships.
Friendship was highly valued in Epicureanism. Close relationships with others were seen as a source of happiness and security.