Why do people allow themselves to love someone that doesn't love them?

How does this happen? It doesn't seem fair at all to the person that gave so much to get so little in return. How can one person feel so "connected" and the other not even care? I am speaking not in terms of love at first sight or infatuation, I am speaking of long term relationships, even marriages where it is clear that one mate loves the other more and shows it, while the other takes and takes but never gives. It doesn't seem like this type of scenario should exist. And who's fault is it? The person that takes advantage of that unconditional love or the person that allows themselves to be taken advantage of?