If we are talking about ‘why’ God exists, we have some interesting questions. Some theologians and philosophers who believed God was a Being or Being itself (or unconditioned Being), such as Aristotle, Plotinus, Aquinas, Augustine, Al-Ghazzali, Descartes, Duns Scotus, Spinoza, Hegel, Anselm, Nicholas of Cusa, and others, believed that God or the Highest Principle exists by necessity (that is, God’s Being is perfect by definition of what he is or by the nature of what he is) and so he needs no higher principle, cause, or reason to explain why he exists, or how he exists.
Neo-Platonic thinkers and mystics however, particularly those like Eriugena and Eckhart, saw more dynamicism in the divine nature rather than static perfection. While God’s essence for these thinkers is still a ‘nature’, it is beyond essence, substance or being, and therefore Being is in some way a creation of God. There is also a strange dialectic between God and what is created, in the sense that God’s nature or essence creates but also created things ‘flow’ by a sort of necessity from this essence. These thinkers argued God is in all things and all things are in God, and tried to use this dialectic to explain how a changeless, infinite source can make a changing universe (this dialectic occurs in Plotinus, Pseudo-Dionysius, Eriugena, Eckhart, and Nicholas of Cusa, and more recently in Hegel, Schelling, and Whitehead’s process theology).
I think a more interesting and fruitful question is asking why does an Absolute or First Principle or God exist, which most of the world’s religions seem to agree on, is rather how this Absolute or God relates to the world and why does He/it make the world, and how we are connected to this First Principle, and if not, how we can have union or communion with it.