When you do something or make a decision based on pleasing somebody else. The important factor is that the direction you took (the action or decision) was taken only to satisfy the other person and not something you would have normally chosen to do. Regret surfaces and you secrectly hold it against that person. You may or may not take it out on them, but it does eat you up inside.
Emily had always dreamed of moving to the city after college, but her boyfriend was dead-set against it. As a result, she never moved there because she wanted to be with him. She's harbored resentment for him ever since.