I grew up in an abusive household and went on into abusive relationships and friendships. More often then not, kindness is used as a bartering tool. In fact, its been extremely rare in my life to have any form of kindness not come without repercussions. I tend to not accept offers from people and it makes me very uncomfortable when people do something for me I may not be able to reciprocate (or when they don't accept it). Because even if someone covers your lunch and they don't accept being repaid I don't dare think it won't come back to bite me in the butt. It always does.
Interestingly enough, I had a situation a few months ago where I could have used my own act of kindness in retaliation to something. I did a HUGE favor for someone (actually two people), it was out of the blue, something I could do for that person, and at the time they were extremely grateful. Flash forward six months and suddenly I am a horrible person for being stressed. It's extremely hard to not shake that persons memory and remind them of what I had done for them not so long ago. It was absolutely huge and it was something I didn't need to do at all (and no I will not elaborate beyond that). I remembered in that moment that regardless of what was currently happening, I did what I did out of pure kindness (and I would do it again) and if were to point that out then doesn't that change my intentions?
I can't tell you how many times I have allowed people to do things for me out of their own kindness and had it turned around on me. So my message to everyone reading this is if you do something for someone out of kindness don't hold it above their heads, don't look for something in return because that's not kindness and it's not how people should treat each other. If something is expected in return talk about it beforehand and if you're not looking for something in return then don't bring it up months later as a bartering chip.
It's also pretty awesome paying your friends in wine when staying over. I mean if they are into that kind of payment because it comes with loads of laughs.