Nevermind, I just watched a good video explaining it. Basically, being necessary is considered great, therefor God must have it, therefor God is necessary.
But it seems to, as I suspected before, that the idea of a maximally great being hinges on the definition of "great." For example: why is omniscience great? Someone with omniscience can never experience the joy of discovery. I could also say beards are great, therefor God must have a beard. As far as I know, there doesn't seem to be an objective standard of greatness. Of course, I could be wrong.
It is an error in logic to conclude God must have what traits humans can dream up. God has His own traits, and these are separated from the creatures’ ability to think about them. It isn’t hard to show men do not even know their own traits, for instance the origins of hatred and enmity, and such entities, failing to know themselves, obviously would also fail to know their Maker. So if you are thinking of something you call “great,” God might not agree, and even should His traits be described to you, you may not comprehend them. For instance God might laugh and say, “I didn’t need nations, powerful people have no borders.” You say that isn’t great, you needed the nations, but admit your power is slight.
There’s also a human-centric fallacy here, if it is supposed humans are superior to God in their joy of discovery. You’re making a terrible presumption that God’s emotions will be something of a parallel to your own, which shows a very poorly developed God-concept as someone greater than yourself. God’s joys like His traits, are His own and not on the bargaining table for humans to grab for themselves. Not even the angels expect to taste God’s joys, but humans were they wise, would crave the angels’ joys. In general God is known only through revelation, not through attempts to superimpose human traits above. You may not accept my ideas of what God is like, but I can show how others are in error.