The old adage "You get the behavior you tolerate" is also true I think. I guess there are a zillion reasons why people a) put up with it and b) lurch from one abusive relationship to another.
But basically I think that pattern won't change until that person starts to gain more self-respect and becomes more assertive about what they are, or are not, willing to accept from another person.

I'd like to think I'm a nice guy, or so I've been told by my many female friends that are looking for a nice guy, but always seem to attract bellends.
They're always complaining about this, and I keep encouraging them to look, not mentioning myself, cos if they liked me I'd rather they discovered it on their own rather than with my influence.

Anyhow what I don't get fully is why they overlook friends that they've known for a while that are sure to treat them right and all, personally I'd be careful not to overdo it because I imagine it'd get boring being nice all the time, but just not being an absolute dick.
I mean, I can understand if they don't want the friendship to come to an end on bad terms should the relationship go sour, but I hope that most nice guys would be understanding, like I am (sorry to be self-promoting, but I do need a boost of confidence every now and again xD) then surely they could just come to terms with the fact that they're better off as friends, having given it a try and remain friends after if they should resolve it without too much shit in the way.

Anyway, my advice would be to look a bit closer to home, so to speak, your perfect guy/girl could be right in front of you and you don't know it yet.

Just my two cents.

Robthe bear and Afrodanny, you're both bang on there - I've also known loads of women platonically who I'd have loved to go out with, but although I've been "a good mate" or "a nice guy" none of them have been remotely interested. Instead they've been attracted to people most succinctly described as "arseholes" - ranging from blokes they know to be married, to the occasionally violent or threatening types.

I'm talking about intelligent and attractive people here as well, who seem to lose all sense and perspective when it comes choosing partners.

It often seems to me that women enjoy a sense of danger in their men, a predatory aspect - which I certainly don't possess..! I've always felt that because I try to be respectful and treat them as people rather than slabs of meat to be used, it prevents them from seeing me as a potential partner.

The old saying is certainly true, "nice guys finish last".

Well I had a friend who had a shitty boyfriend. I got her through it. One year later. She was another boyfriend who was shitty >_>. I dunno I think some girl's are just drawn to bad guys.

Some of the boys are attracted to the bad guys too......that's why gangs have leaders and followers, they also see something they're attracted to as well.......and I don't mean as lovers but more characteristics/behaviour.... and so on.