Consider the following. We humans are social beings. We come into the world as the result of others' actions. We survive here in dependence on others. Whether we like it or not, there is hardly a moment of our lives when we do not benefit from others' activities. For this reason it is hardly surprising that most of our happiness arises in the context of our relationships with others.
Nor is it so remarkable that our greatest joy should come when we are motivated by concern for others. But that is not all. We find that not only do altruistic actions bring about happiness but they also lessen our experience of suffering. Here I am not suggesting that the individual whose actions are motivated by the wish to bring others' happiness necessarily meets with less misfortune than the one who does not. Sickness, old age, mishaps of one sort or another are the same for us all. But the sufferings which undermine our internal peace -- anxiety, doubt, disappointment -- these things are definitely less. In our concern for others, we worry less about ourselves. When we worry less about ourselves an experience of our own suffering is less intense.
What does this tell us? Firstly, because our every action has a universal dimension, a potential impact on others' happiness, ethics are necessary as a means to ensure that we do not harm others. Secondly, it tells us that genuine happiness consists in those spiritual qualities of love, compassion, patience, tolerance and forgiveness and so on. For it is these which provide both for our happiness and others' happiness.
[Ethics for a New Millennium, by His Holiness the 14th Dalai Lama]