When you feel great delight in someone, meeting their needs and getting their gratitude and affection in return is extremely rewarding to your ego. At those times you may be acting more out of the desire to get that love and satisfaction yourself, rather than out of a desire to seek the good of the other person. Kierkegaard observed, you may not be loving that person so much as loving yourself. 
Timothy J. Keller, The Meaning of Marriage: Facing the Complexities of Commitment with the Wisdom of God
gospel, marriage