Tis a simple question, no? If Marshall McCockNswag doesn't want to date you, why is he still making an effort to be your friend?? Surely any man worth his weight in baby batter would not go through such efforts to hold on to a female friend that he doesn't engage in coitus with… Right? Not true, of course. There are a number of different reasons why this is. There's a post by the always insightful, sometimes super-heroic, Jeffrey Platts where he discusses the theory that your soulmate isn't going to be anyone you know. I suggest you read it. The article makes solid points that I think we sometimes forget and it plays into why a guy will remain your friend, even if he's not wanting to date you.
You see, it might be that you're unavailable or you carry that air of unattainable-ness (which you're likely unaware of) that strings a man along. It's in our nature to hold out hope for the things we want. For the sake of this article, I'll keep the “hope” focused on interpersonal relationships.
Some of us live in clutter. No, not like the folks on ‘Hoarders;' we keep mementos; we hold on to sentimental keepsakes; inanimate objects that remind us of a particularly fond memory. We keep this same clutter in our relationships, too. That is, we keep people around that we have an interest in (whether you want to admit to it or not) because it's comfortable or, simply, because we admit our adoration for a person and secretly hold on to the hope we might be with that person some day.