Well there are 2 types of people that think they're perfect: The ones that are really vain and drool over their own reflection in the mirror (like this guy I know at my school)
And the ones that are really insecure inside so it's not that they think they're perfect, it's just that they try so hard to be and convince themselves they are and that everyone likes them in their own little world.
Sometimes people like to be perfect to make people like them. Then there's the ones who have never ever had a trial. Your right about the nobody's perfect apparently the people that think there perfect don't know or never heard (maybe) that nobody's perfect and they don't want to. Also people just are in secure and like being perfect so they say they are. I hope I have made myself clear and have helped you understand.