Friendship is the flip side of loneliness: when you have people you can talk to and pull pranks with, you feel less lonely. (And let's be clear: no one in Uglies wants to be a hermit—everyone wants friends. Even the people who ran away to the woods—Maddy and Az—want other people to hang out with.) But as sometimes happens when people grow up, friendships change. It's an unfortunate truth that some of your ugly best friends might not be your best friends when they become pretty (at least, that's true if your name is Tally). Which raises one serious question: why are these characters friends in the first place?
Friends have to be equal. That's why pretty Peris can't really be friends with ugly Tally.
In Uglies, friends are an important part of life, but finding your own identity is more important.