I think a big part of what causes resentment in relationships is when you are constantly going out on a limb, making yourself vulnerable, and/or doing a majority of labor (emotional or otherwise) and you don’t feel the other person is “stepping up” in the way they should. This causes a sense of “unfairness” or general imbalance, which leads looming resentments (at least for me). It can also make me resentful when I don’t feel like there is a forum for me to express my feelings and be heard.
Not sure where I was going with this, but there it is.