"He is the leader of the Guardians Of The Galaxy"
Uh, at what point in the films do they decide that Peter is their leader? I must have missed that. I think it's made pretty clear that they decide everything by consensus, which is part of the reason they're constantly arguing with each other.
I grant you, he's…

I would also posit that having powers on that level and of that type could certainly cause you to have a mental illness, particularly if everyone around you were telling you that you were imagining things. I can't imagine I would handle it well if I were him.

All together now:
I like New York in June…how about you?
I like a Gershwin tune…how about you?
<sniffle>

Just for a start: Robin Williams is not believable as an angry man, much less as the angriest man in Brooklyn. Mila Kunis is not believable as a physician. And it gets much, much worse from there.