Good question, but the answers are many and varied. I would guess the most relevant reasons would be:
>that acceptance by others validates ones need to overcome feelings of low esteem.
>being brought up in a familial situation that always looks at the negative side of things makes positive feedback a major goal in life.
>being told as a child you are ugly, stupid and unworthy
>but the most popular is being told that people in general are sinful, evil, and especially the notion that thoughts are as sinful as deeds (religious teachings).
Popularity suggests higher self worth, so excelling in sports, school, or ones career also validates that self worth, and acceptance fulfills this craving that many people have in life.