I'll quite happily give up my time to make others happy, because doing so is its own reward. However, when that generosity is expected (people repeatedly asking for favours, etc...), that's when resentment starts to build.
So you see, for some people (myself included), I'm happy being 'nice' without expecting anything in return, but I'm not happy for that to be taken for granted.
Someone that cares about their own stuff, is confident etc, both provides an example to follow and implicitly projects a moral judgment that it's OK for others to live how they do.
Someone who hurts themself (question of degree here maybe) to be nice implicitly projects the moral judgment that others should do the same. People don't really want to be like that; being around the person makes them feel guilty, or perhaps they just don't want to appear disagreeable by not reciprocating; they avoid the person except insofar as they might actually want the help being given.
Unfortunately (especially so for people who honestly hate themselves and would never explicitly recommend that others emulate their own self-deprecating behaviour) that moral implication - that arises from the reflexive belief that everyone is in control of themselves and so ultimately wouldn't do anything they don't approve of - is not entirely avoidable, since it rests partly in the eye of the beholder.
The question is what would happen if the jock also helped the girl in the library? Would that make him more or less attractive because he had become attainable?
If being helpful gives you one point, being confident gives you a hundred. You can be a confident and a bit selfish (but not mean, or a jerk) and still be at around ninety-nine points, which is a lot.
I can't give many examples without the context, because they won't come through correctly and people will just think I'm being mean.