This reminds me of a passage from Shantideva. In the passage, he says something along the lines that true happiness being found in rejoicing in the happiness of others. As a parent, I find this to be particularly true. When I see my daughter full of joy, it is such a cause of happiness for me.
I would like to say a few words about rousing the mind of awakening, the mind that is directed toward supreme awakening. This is the Tibetan way of practicing the excellent dharma. What is the reason for this tradition? Generally, in the Buddhist way, if we repair our motivation at the start, our conduct can become pure and correct. If we do not repair our motivation at the start, our conduct cannot become pure and correct. For that reason, we need pure motivation. What pure motivation do we need? Generally, we should undertake activity that benefits ourselves and others rather than activity that harms ourselves and others. If we cherish others more than ourselves, that will serve as a cause of what benefits both others and ourselves. Therefore, let us exert ourselves at activity that benefits others.--from Essential Practice: Lectures on Kamalashila's Stages of Meditation in the Middle Way School by Khenchen Thrangu Rinpoche, translated and introduced by Jules B. Levinson, published by Snow Lion Publications