The right-wrong debate.

People tend to think that they know what is best for themselves and usually project that onto other people whether right or wrong. You see it in the media and in school and at work. If your views clash with someone else, the other person will try to change your opinion or you will try to change theirs. Even with research an opinion is at least partially subjective and no one likes to be told they're wrong. If you think you're right all the time (even if you're not) then you probably think you're more intelligent than the rest of the population. If your opinions and knowledge are right (to you) then why wouldn't you be above average? 

Admitting that you are average has connotations that you are not unique because the majority of people are at the same level as you. You're not a leader, genius, prodigy-- someone to be proud of-- but you're also not a leech on society, you're not making the world a worse place to live. You're just there, consuming and contributing in no special way. 

People like to feel good about who they are, they like to think their opinions matter, and if their opinions matter; then they must be above average because their opinions have worth above the masses.