Sometimes people can't return a favor, even if they want to. This is usually because they don't have the skills or resources to, or they do have the skills/resources but there's no real opportunity to return. (An example of the second situation: You spend an afternoon help a buddy fix up their car. Your buddy wants to return the favor and is capable of doing so, except that your car isn't broken.) So, all else aside, it's best not to expect returns for favors in general. You're too likely to be disappointed. Do it for the sake of doing it.
That said, though if you've helped someone again and again and they never seem to even try to do anything back, they're probably taking advantage of you and you are completely in the right for being upset by it. I've been in a lot of relationships like that, and they suck. There's nothing wrong with wanting to fix them or get out of them; there's nothing wrong with resenting being used.
Still, that's not really a good reason to just shut down any sense of altruism. There's a lot to be said for helping folks, even if it just for the sake of helping folks. Personally I try to give strangers the benefit of the doubt and help them as I can. Friends and family, eh, you just kind of have to gauge it over the long term. Again, you can't really expect a return for every little bit, but if it's starting to look like they're just manipulating you and have no intent of ever helping you in return, it's okay to want to do something about it.
