From a young age we are taught to share and we are taught that happiness is important. You put those two together and can find yourself spreading cheer through the gift of giving. Watching somebody smile as you hand them a gift immediately allows a sense of happiness to overcome you.
Social psychologist Liz Dunn and her colleagues performed experiments on several groups of people. In one controlled experiment they handed an envelope of money to students from the University of British Columbia. The students could either spend the money on themselves or spend the money on someone else. Both options had a 5 pm deadline. As discussed here, it was reported that the students who spent the money on others were happier than the ones who spent it on themselves. The euphoria that overwhelms you after making someone else happy is incomparable to say the least. It is amazing how the human mind works. We can be selfish and selfless all at the same time. We know that by providing for others it is making them and ourselves happy all at once.