The label nice guy is not something positive.
A nice guy has high morals, is quite sensitive, respectful and well-mannered. He does not work the dating market the way nature intended it. Instead of fluttering from flower to flower like a butterfly, he targets one particular girl and courts her for a long while. Of course this almost always fails, but the nice guy has trouble letting go of his often mentally idealised target of affection. Rejection is answered with confusion and misunderstanding. After lots of rejections, the nice guy can metamorphose into the bitter loner.
I am a specimen of this sad life form, so I've had some experience analysing my own modus operandum. I've been quite bitter for two decades after some particular nasty rejections in high school. I've grown out of it, realising after all those years that the rejecting girl does not control her feelings any more than the nice guy does. (She does however control her actions, but I don't want to drag this into the analysis of the evil bitch, another life-form in the love eco-system).
If you can get a nice guy to love you, you have a mate for life. He's been hurt so often that he'll never want to inflict this suffering onto someone else. Sometimes however he might carry some baggage from previous botched courtships.
How does one recognise a nice guy? They are rarely handsome. (Handome guys have easy pickings on the dating flower beds). The nice guy acts a bit corny: he holds doors for women, proposes to help out more than he should, apologises a bit too often. Mothers like nice guys a lot. They always wish their daughters find one. This, I can tell you, is particularly frustrating.
The reason why girls are not attracted to nice guys is because they emanate some vibe of desperation. I've never understood this. It is very counter-intuitive since nearly all love stories are about some nice guy exerting substantial effort to court a love interest. To me it seems like the more I show a girl I care, the less she wants me. (It's this observation that lead to the development of the highly succesful pick-up artistry).
I think that the nice guy who succesfully connects to his coveted girl, at least for a few years lives in absolute bliss. I have yet to reach that stadium. For almost three years now I've been hoping to ask the most perfect girl this side of Constantinople out on a date. I have to wait because the odds are terribly against me. They might however never improve. Do you notice the absurdity of this situation?