When I look at every nice thing I've ever done for someone else, I think the only motivation has been to have something good happen for me in return. It's not something that's a transaction between me and someone else though; I do something nice, and continue to do nice things with the expectation that others will reciprocate when it behooves them. It's a mutualistic relationship I extend to others. If I continually offer kindnesses and give of myself, and someone takes advantage of that without reciprocating when it wouldn't be that hard, or when I really need the help, I recognize that it's not a good thing for me, and tend to gradually disconnect myself from that person.
I don't view that as manipulation at all, but rather a sign of a healthy respect for others, and for yourself.
I rather... detest karmic balance morality. Doing nice things with the expectation of nice things in return is certainly good from a productivist standpoint, where optimization is desired. IMO though, friendships and relationships aren't contracts where you expect things from each other. You help them because you want to help them. A good relationship is an altruistic one, both ways.
Ack, maybe I'm not conveying things well.
I don't do nice things for others because I expect the universe to repay me in kind though some underlying cosmic balance. I know that every good thing I do for anyone else makes things better for them, thus improving their ability to make a positive impact on the world, and thus helping me and the things I have a stake in. Also, I hope to encourage an attitude of helpfulness and reciprocation in those around me, and help them out when I can, in hopes that they'll do the same for me when I need it. Thus, being a nice guy.
On a related note, I don't really consider Romantic Relationships and Friendships separate things. My monkeysphere is inconveniently about the size of a planet, and I can't really construct a hierarchy of who is more or less important to me. I sometimes worry that the relationships I've been in have failed, in part, because the love I show a significant other is much the same as the love I show most people in my daily life. Everyone, even the people I detest, are important to me... I want them to be mentally and physically well, constructive, and contented people. I wonder if that's made my romantic partners feel like they're less important to me or something.
Hmm. Perhaps something similar could play into what we perceive as the Nice Guy phenomenon as well?