For what it’s worth, I deem: ‘Why does God exist (i.e., “stand forth”)?’ = ‘Why does God create?’ = ‘Why does God exist as Creator and Lord Jesus Christ?’
The last of these questions is really the most sensible formulation, at least in my opinion. I can’t answer the first, why He exists, beyond asserting why He can’t not exist, stating that existence, or standing forth – presenting – already entails something potentially stood-forth-to/in, and therefore implies something prior, grounding and allowing relation. And any sort of relation requires a diversity, a manyness – at least twoness – that runs contrary to simple oneness; relation necessitates division, a split, side(s).
‘Relate’ = ‘re-’ (back, again) + ‘latus’ (used as suppletive pp. of ‘ferre’ (to bear); and, coincidentally, a Latin word for ‘side’ is ‘latus’ (carried, borne), while ‘ferrum’ is a Latin word for something solid that splits things, ‘sword’. Oh, and di-vision* is just double vision – from L. videre, meaning either to see (sight; knowledge) or to separate (se = self; root par- = equal) – or knowledge of two, which implies two things, i.e., at the least, two “images”/senses of the same thing. Relation, be it of the visual or epistemic sort, somehow stands as a unity among, at minimum, two units, co-responding acts of relating.]
And so an act or process of relation itself must consist (pace ex-sistere, con-sistere, meaning to come/cause to stand together) of, like the visual relation, at least two units/participants within it, somehow making it up, or in (yet) other words, creating it. ‘Why does God create?’ asks for His intention, what He has in Mind. If, as we believe, God is Essentially One, then the intentional relation is impossible for Him unless the very relation is creative, an act of creatio ex nihilo (though perhaps more accurately dubbed creatio per se (through itself) or creatio… intima (inmost)? interneca (between the center; from within)? intestina (inside/internal, esp. w/in or re: a body)? His relating to Creation is His creating; and His relating is also His knowing, which is traditionally conceived as actively causal, not passively receptive, knowledge. Relating, creating, and knowing are all simultaneously one in God. As it turns out, the knowing relation to creation need not presuppose creation, because it ontologically proceeds from it, much in the same way a line segment can only be defined by postulating two independent end-points, proceeding as an infinitely divisible connection between them. The intuitive answer to the first question appears to conflict with the paradox of the second.
He is what (Whom) He is, because He knows what He knows; moreover, He is His Knowledge, a perfect – thus absolutely desirable and worthy of love, or good – knowledge, which might describe why and how He creates. He has Himself in (as) Mind; He intends Himself. Love (Spiritual) is constituted by both a first knowing (Intellectual) and posterior desiring (Volitional), and love is most perfectly creative, generative, and productive. God’s willful knowledge of Himself causes, or is identical to, His being One with and conceiving of Himself, that is, begetting the Son of God, Whom is One in Being with the Father. Thus were there Two in One, viz., two in one act of causative self-knowledge, then naturally making three in one, persisting to spiral forth ad infinitum like the Fibonacci Sequence (1, 1, 2, 3, 5, 8…). One united infinity of infinitesimals.
So why does God exist as Creator and Lord Jesus Christ? It’s only fitting that God’s ultimate existence within His vast creation comes in the material form of Man, Imago Dei, directly at the gaping center of it all, suffering at the most proudly delicate, side-splitting, and twistingly divisive of spots, a speck within a possibly infinite universe between heaven and hell – yet maintaining rational dominion over and among his earthly home and its inferior inhabitants, (re-)claiming the peak with uniquely specific diversity, enlightening, industrious knowledge, and freely reproductive conception.
*Vision (properly) occurs in man as a perceptual synthesis of two distinct images, as a result of his naturally possessing two eyes, thereby seeing in terms of a brain-interpreted unity of two (slightly) differing perspectives toward one self-identical (equal per se) object. One advantage of having two eyes is that a greater amount can be seen of any divisible, material thing than would have been possible from any single point (i.e., 3-D, stereoscopic vision); another is that sight remains possible, albeit in limited capacity – a la the Fall – in the event that one of them is injured/blinded. For one reason or another then, it seems, man’s knowledge is only mediately singular and unified after differentiated sense-data; it’s always, like himself, a unified composite permitting of abstract analysis into self-insufficient parts, or ex post facto, ex uno derivable co-principles.