Humans get bored with monotony, to counter monotony we invented holidays, this particular holiday is all about exchanging gifts with our kin by placing them under a pine tree, sometimes we also hang long socks about our fireplaces and fill them with lesser gifts. It was originally called Saturnalia by one religion and was co-opted by another religion (as a mechanism for gathering followers) which renamed the day Christmas. Nowadays the whole event is pretty much secularized and celebrated by the non-religious as well. We have a bunch of such holidays, like the one where we dress up as mythical creatures, the one where we periodically light a bunch of joined candles while giving and getting gifts, or the one where we fast during daylight hours for about a month.