Nice Guys live by "covert contracts." This is the unspoken agreement that says, "I will do X for you, so you must do Y for me, but I’m not going to tell you what Y is." When others fail to reciprocate, the Nice Guy becomes resentful. The Dark Side of Being "Too Nice"
Doing favors with the silent expectation of receiving something in return (love, sexual affection, or approval). No More Mr. Nice Guy