My mom says that her dad carried around with him grudges forever - at age 80 he still bitched about things from his youth; she (although she wouldn't admit it) is the same way, as am I. I can remember very clearly things from grade school that I could still get pissed off about even though they're completely irrelevant to me today. (A corollary of this is that I also remember almost all of the mean things I've ever done to people.)

One of the things that really, really angers me are gifts-with-obligations. The "I gave you/did for you [something], so you owe me" sorts of things. If you're going to do something out of generosity, then give it and be happy. If you want something out of it, then say so at the beginning. Making a transaction look like something it's not is very very low in my book. Admittedly, this can happen often unintentionally - e.g. people who get involved with each other are notoriously bad at times at reading what the other wants and is expecting.

It's this sort of thing which really irritates me about birthday and xmas gifts... and especially "favors" which ain't.