I think that with love the butterflies stay, and eventually the more time you spend with someone and the better you get to know them things just work. Eventually, you get to a point where you see the person for who they really are, and if they're important enough to you; you'll forgive them for what they did to hurt you. If they didn't mean that much to you you'll eventually move on and forget them. Some people come into your life that you'll never forget, whether they affected you in a good way or not. I don't think it's possible to really fall out of love with someone if it's intense enough. Although, I don't have any proof of this.
Well if you know its not a good move to get into a relationship with this person, you should try weighing up the pros and cons. It depends on the actual reasons behind the walking away.