As a girl I can't offer a terribly objective opinion about it, but I would imagine it to be an extension of machismo. Because a scar represents having participated in something "cool" or "dangerous" (which could easily be classified as stupid, in some regards). In other respects it represents toughness, as in "yeah, look at this, I went through this and I'm still here and badass", and that is pretty important for even the most unassuming of guys. And, well, it's instinctual. Attraction is as much instinctual as it is intellectual for we humans, and a potential mate could see a scarred individual as someone with fortitude and power and security, i.e. good genes. It's as much an attraction of mates issue as it is a dominance issue, like "Yeah, I'm a badass, check out what I've been through, what have you got, punk?!".
That's just from my (admittedly limited) understanding of basic psychological precepts as well as what I've observed (and in some cases, felt). Our instincts lead us to make judgements on each other and on our society, and those same instincts cause us to come up with ways to be judged favorably by our peers.
For guys, toughness...i.e. scars...is a way to do exactly that.
Just my opinion though
