Sometimes it's easy to tell when someone has your best interest at heart, but sometimes it's not so easy. At least in your mind it might be, because you want them regardless of whether they really want you or not. It sounds crazy, but it happens and it happens often enough, to enough people.
It can happen when you're young and infatuated with someone to the point where you think that you can carry the love far enough and long enough for the both of you.
Loving someone who doesn't love you, in hopes that they will turn around and start loving you? Does it make sense? Does it have to? This happens in more situations than you may think. It's the ugly part of one-sided relationships.
Many times when people are really really bitter about a relationship ending, it's because this type of situation didn't pan out. Believe it?