You Get To Choose Who You Like
Obligation is a huge part of our culture. We feel obligated to people due to culture, familial relationship, friends of friends... We do nice things and favors for people who, quite honestly, don't deserve them. It might be because we don't want to make an enemy at the office, or it might be because it's your betrothed's mother or sister or brother and you just have to make nice. But that's not really what I'm talking about here (but the point still applies).
There's this element to some peoples' friendships with others that depends solely on how much they do for them. And to these people, I say that a friendship based on how much you do for someone is no friendship at all. And I am willing to bet quite a huge sum of money that, if you're one of these folks, you've convinced yourself that this "friend" is a nice person and you like them, despite the disappointment you feel every time you need something from them and they're nowhere to be found.
You get to choose who you like. You get to walk away from this situation if you want. You don't have to keep a relationship going with anyone who doesn't work as hard as you do to do the same. When you stop going the extra mile for someone who doesn't do the same for you, you suddenly figure out that the only consequence of removing that from your life is that you no longer have the responsibility of doing things for someone else with no reward.
Or, in much shorter terms, fuck that shit.
It's like the rest of everything else in life, like going to the gym and being in shape, or getting a better job, or chasing your dream. Give yourself permission to concentrate on yourself, and you'll end up much happier.