We, as humans, find happiness in having sympathy with our fellow humans. It is not our of pure self interest that we feel these feelings. The pleasure also doesn't arise purely from whether the reactions of others please or flatter our own feelings. What is the source of pleasure is the correspondence of sentiments with other people. The sharing of similar feelings is what matters.
Sympathy enlivens joy and alleviates grief.
We are more likely to share our grief with others rather than our joy. When a person shares their sorrow they have to relive the pain. However, in the act of this sharing the pain is at the same time lessened. What hurts people is if their suffering is not ignored. The reason for wanting to share our pain more than our joy is that joy in itself is enough to make the heart happy.