Why does Bellerophon believe he can be equal to the gods and what does Bellerophon forget?

Bellerophon believed that he could be equal to the gods because he had many successes, such as killing the Chimera. He was also very ambitious.

Bellerophon forgot that this kind of thought really ticked the gods off, and Pegasus knew that. Therefore, Pegasus threw him off and wouldn't let him fly.