[calineczka]

My personal theory: same reason but scoped to a limited domain. You can be a generally happy and successful person but a fanboy of an athlete because you've never achieved much in sports. You want someone to succeed because it resonates with some unrealized potential in yourself.

[cabalamat]

That makes sense. To the extent I've ever been a fanboy, it's because I wanted to emulate that person in some way.