We celebrate Christmas because we need to
Someone asked me, during our Friday night drinking session, why Jesus' birth is celebrated at Christmas. I never have a ready-made answer for that. I don't think the reasons for His coming are to be found in the Nativity myth - that was a confluence of two stories that were probably added because there were still people alive, when the Gospel was committed to writing, who remembered Jesus as someone who came from Nazareth, not from Bethlehem.
For those of us who believe, Jesus' coming is a very significant and profound moment in history. God became incarnate as a human, and took on our struggles, our weaknesses, limitations and vulnerabilities, so he could reach those who needed Him. And, reading the Gospel, I get the impression that Jesus lived in a time and place where most people were despondent towards organised religion and estranged from what it preached. Maybe people generally were no more or less religious than we are today.
I explained that the pagan traditions around the Winter Solstice were later merged with the celebration of Jesus' birth, as cultures became Christianised. It wasn't a point I made dismissively, the way an uneducated atheist would. That aspect of paganism is something I deeply appreciate, as someone who feels more vulnerable to depression when it's cold and wet outside. Winter conditions would have presented a threat to our ancestors who weren't so insulated from nature, so the Winter Solstice marks the advent of that which brings life.
I think all that is lost under the commercialism, the barrage of advertising and the mad rush to buy as many things as possible. But we still have that subconscious need to celebrate a demarcation between the period leading up to the darkest moment of the year and what follows. If we didn't, we would merely be living through an unbroken period of winter, and that would be depressing indeed. That's why we traditionally counter the cold and darkness with lights, decorations, drinking, partying, etc.