It is well known (for various definitions of 'well') that there is an attraction between women and 'assholes' (
case study). There is, however, a just as popularized (if not more so - it has been in the
media for several hundred years) but not as acknowledged. It is the White Knight
Syndrome.
By the very nature of the nice guy, there is a romantic notion of saving the damsel in distress. This notion is deeply rooted in the minds of guys - from Dudley Do-Right to almost every work of fantasy. We see the good guy rescue the damsel in distress from the evil warlord, the dragon, and later the iron dragon of the train. To many, this is the image of what love means - it can be just as disabling in finding what actually constitutes love as someone who believes that being hit is the proof of love.
Somehow, something triggers the damsel in distress. It may be real, or imagined by the white knight. Given that the majority of these males are single (see above case studies), it is quite likely that the modern day white knight shall become enamored with the woman and it makes the 'quest' all the more important to the white knight.
Unfortunately, it is difficult to get rid of a white knight. The most effective method is to demonstrate that that the damsel is not in any distress at all. This can be difficult at times without hurting the feelings of the white knight (often the nice guys are also the sensitive ones too). Still, the enamoring will continue for some time (be it infatuation or love - the terms are left to those who wish to argue over them).
Much of this information is first hand. Yes, that is right, I do suffer from White Knight Syndrome from time to time.