If you're not in the mood to hit the town and whoop it up, then you're not going to be much fun doing it, are you? This is the logic you should use when you tell your friends that they should count you out of the latest big social scheme. You're feeling more introspective right now, and the idea of going out and making small talk with strangers is not striking you as the recipe for a good time. You need to do what you want, not what they want you to do.