Naturally speaking it is the time of year when we have the shortest day. Everyone experiences that.
In harsher times it is the time of year you would hope to have enough to survive the dark and the coming winter.
If one had enough then one would feel glad about this, but it was also the time of year when there were people without, and surely by helping them survive by giving is where the gifting really comes from?
Indeed. It's also worth celebrating the fact that the days are going to start getting longer again, plus as the darkest point it's the most useful time to try to make things a bit more cheerful with decorations etc.
Religion not necessary to see that it's a great time for a festival.