The blog Facts About Religion asks, “[W]hy is Gods nature like A, and not like B? What determines Gods nature? (And if the pat answer is ‘well God determined his own nature to be Nature-A, not like Nature-B’ why did he determine Nature-A instead of Nature-B?'”
Undoubtedly there are better answers, but here is mine:
God must be as he is because if he were to change in any way, he would no longer be perfect, and therefore no longer God.
Suppose you had drawn a perfect circle and someone asked, “Why does the perfect circle have to be perfectly round? If it’s because that’s its nature, then why is it the nature of a perfect circle to be perfectly round instead of imperfectly round? or perfectly triangular?” Quite simply that’s just what a perfect circle is: It’s the nature of a perfect circle to be perfectly round instead of imperfectly round, because if it were imperfectly round, or if it were triangular, it would not be a perfect circle.
Similarly, if God were not e.g. perfectly powerful (not limited by anything outside himself) or perfectly actual (not dependent on anything outside himself), he wouldn’t be God.
In what way could you change God such that he would still be perfect? This is like asking, in what way could you change a perfect circle such that it would still be a perfect circle? You can’t. Any change in the circle (that is, the circle in its essence as a circle, disregarding what color ink it’s drawn in, or how large it is, etc., which are accidental, not essential properties of a circle) could only detract from its perfection and therefore make it no longer a perfect circle but an imperfect one.
By the same token, any change in God could only detract from his perfection and would therefore make him no longer God.