Why would a guy tell you he likes some other girl when she doesn't even care?

Maybe he likes you also and they're trying to get you jealous. Or he's faking the fact that he likes the other girl and he ONLY likes you and he's (again) trying to get you jealous. OR, his friends think you like him and they're trying to get you seriously annoyed/mad or trying to get you to admit it. Those are the only reasons I can think of.