Its becasue love is more than a friendship. Its a soul bond between two people. Therefor you get more deeper and more emotional the more you love someone.
The truth is, when we "lose" someone, like when they break up with you, your not really losing anything. In fact, your gaining the experience that you have earned by being in the relationship. That's what relationships really are. Every single relationship a person enters, they are going to gain some kind of experience in dating, love, ect. Whether or not the relationship works out, that experience is all that matters because if it does work out, then you haven't lost anything. And if it doesn't work out, you'll use that experience in the next relationship.
My theory is that this cycle repeats itself until we find the one we truly love. In which case, our past relationships have already given us all the experience we need to make it succeed. Of course then comes of the peronal opinions in the relationship, or the accidents, or the things that we don't expect. In those cases, that's where your skill comes into play. Your skill of loving the other person, understanding them, knowing what it feels like to be something different, being someone different ect.
So no, you don't really fall for the "wrong" person, your just building it up until you find the "right" person. And we let ourselves be put down over and over again because we know that eventually things will get better, it always has and it always will. Its in all of us to know that if something is going wrong, our bodies have to do something to fix it which brings a challenge in front of us. Challenges make us all stronger.
We let it get to that point because despite how we think of our "partners", they are still another human, and its in human mind to let others be who they want to be. We will let our partners do things they want to do, just because we don't want to upset them, we want them to be free, we want them to be like ourselves, and therefore, eventually they are bound to do something bad, or stupid or what have you to the point where we can't put up with them and we leave them.
Its just because were humans, nothing more, nothing less.