But why? Why is it that spending our money on others—prosocial spending—makes us happier?
It's partly because giving to others makes us feel good about ourselves. It helps promote a view of ourselves as responsible and giving people, which in turn makes us feel happy. It's also partly because spending money on others helps cement our social relationships. And people with stronger social ties are generally happier.
The consequences of this notion can work in a circle. Not only do you want to buy gifts that bring happiness to others, it will make them even more happy to know that they gifts they give you are treasured. Link