1) little children [or even big children & adults] can spend their LIVES striving to be just like that person. they lose their own identity and conform to whatever that person is, embodying their entire personality, style, everything
2) people worship celebs like they're God! NOTE: CELEBRITIES ARE NOT GODS! they are normal people. they take showers. they eat dinner. they check their email. heck, they are even known to take a nice crap from time to time.
3) following their lives is simply a waste of time. most celebs aren't willing to give their fans the time of day, and spending hours on end trying to find out where your favorite celeb is going to be for their next book signing or meet and greet for a 30 second meeting they'll forget 30 seconds later is not worth it.
4) they also tend to be just plain cocky. even though they're not God, they have that "greater than God" complex. and then it just goes back to #1, where people act the same way because that's how their favorite celebrity acts.