I think it's because as human beings we desire to feel socially included, and society has ingrained getting plastered as the thing to do when celebrating something. Which is an odd concept when you think about it...
People are different, and some will be able to drink and know where to stop. I envy them, but I know that if I drink, I run the risk of getting so plastered that ill end up ruining everyone's night and upsetting people. Try not to think about Christmas too much, it's a long time away yet, take each day as it comes, you could feel completely different next week let alone in 2 months time
You being clean for 3 weeks is commendable so well done, my mum is a dependant alcoholic so I understand how difficult that is. Keep it up and I'm sure it will become easier.
K
Xx