He has been resenting his parents for favoring his younger brother.

resenting 是动词 resent 的现在分词形式。resent 由前缀 re-（表示"再次"）和动词 sent（send 的过去式，意为"发送"）组成，字面意思是"反复发送情绪"，引申为"对...感到愤恨、怨恨"。 resenting 在这里表示"持续怀有怨恨情绪"的状态。