Pride

The fact that one minute you could be a "nobody". While you are that nobody you have your circle of friends the girls/guys that you have crushes on and they have little crushes on you and so forth. You date them, you hang out with your friends... Cool.
The next minute you become a movie star, a rock star... whatever. Then you start to fade away from the people who care about you. You drift away from that person you truely love. Because you have to experience life. Try new things... Make new friends.
I think its so incredibly stupid how when you don't have that camera recording you or that guitar or microphone in your hand, people don't look at you twice, the second you get one or more of those things. You are special to everyone. And you begin to forget the people who helped you get there.
You have your groupies and your fans there but they aren't going to support you if something happens where you can no longer be "special". You know who is? The people who TRUELY love you for who you are and what you believe in and will support you in ANYTHING you want to do. Even if it means giving up your career to go do something else. Like become a Dr, make a family or become a teacher.
Bottom line... Don't cut the people who truely care about you out of your life. You may need them someday and if you shun them for too long, they may not be there in the end.