The biblical writers never raised the question of why God is as God is or why God does what Goes does. They encountered God in their history as a providential, glorious, loving, and holy being, who came to be seen as the creator of heaven and earth. The universe could be accounted for by tracing it to the will of God, though the prophets never claimed to understand that will in more than the tiny part of it which touched their own destiny. The question of what could account for God was never raised. It remained an ultimate mystery.
It is clear, however, the nothing other than God can account for God. Either God cannot be accounted for – which makes the divine existence and nature something which just happens to be the case – or the divine nature accounts for its own existence. To “account for” is to give a reason; thus the reason for God’s existence must lie in that existence itself. To that extent, a reflective theism does seem to point to an idea of a self-explanatory being, which is what Aristotle was seeking to articulate.
Aristotle sees that the best reason for the existence of anything is its goodness: “the real good is the primary object of rational wish.” If I ask, “Why should X exist?”, a good reason is that it is intrinsically valuable that it should exist. The good is that which can be reasonably desired; and it is good in itself that there should exist a state which consists in the contemplation of supreme beauty and goodness. This is the best of reasons for the existence of a being of supreme goodness, namely, that its existence is supremely desirable, not least to itself.
From chapter 8 of Religion and Creation (Oxford: Clarendon Press, 1996), p. 195.