It's a hormone thing I'd say. Not all teens think that, but the ones that do probably are just happy to say they have a "girlfriend," and they can't imagine themselves with anyone else. I thought this way with my friend Kate. I used to think she was perfect, and that we were meant to be together. But she broke up with me because she felt our relationship with each other was ruining our friendship. We weren't as close as we used to be. I'm glad she did that!