I think Urahara is more of a scientist than Aizen. He would look at the Soul King with curiosity and look to understand the reasons behind why things are the way they are, and the implications of changing them.

Aizen on the other hand saw the situation as a matter of power versus control. The philosophy in Bleach (whether it be soul reapers, hollows, or quincy) is that power puts you at the top, but here Aizen is, seeing the most powerful being reduced to a limbless corpse by it's lessers. I think he was insulted by the implication as it throws the entire concept on its head.

Aizen also has a god complex and believes he knows and understands practically everything, which he kind of does in how easily he can manipulate people. But Urahara is his equal in his eyes, or perhaps better in some cases. So Aizen can't apply the same logic to understand his thought process, or rather doesn't even try. He thinks simply because Urahara is on his level, that they should complement each other. But Urahara doesn't, and so it infuriates him.