Human beings are born in a community, we have close interactions with each other. No one can live alone for all his/her life.
Life has always been a relationship between taking and giving. What can be taken or given could be emotional or mental help. Humans support and share with each other the joys and sorrows, the happiness and the suffering.
That could be visible matters. When you get stuck in a financial trouble, you have your family to help you. Or when you have a problem at work, you are supported by a colleague.
In that interaction of giving and taking, the givers are the happiest ones. Because the givers are rich in emotions, they are ready to share with others.
Because the givers are those who has a warm, tolerant heart, they know how to live for others. They understand that what they have for themselves will be lost, but what they give away last forever.