They choose to be low value when they get bitter about their assumption they are low value.

I'm glad I had someone I wasn't attracted to pursuing me early on because it led to the realization that giving in to that bitterness would just seal my fate when I was feeling down about rejection. Part of the bitterness was wanting someone to say, "hey, no, that's not the case" and date me to make me feel better, but from experiencing the other side of it, I knew there wasn't anything she could have done to make me into anything other then friendship and the more she pushed, the less I'd be sympathetic, not the other way around.

Things didn't turn around right away when I realized that, but it was an important part of the "don't be unattractive" rule. There's more to it, of course, but being whiney and bitter is pretty unattractive to most people, I'd guess.