Many factors influence people who become activist and drift away, but a few seem central. The left is no fun. For most, this is not an appealing thought. It isn't that the Left doesn't know how to party. It is that by virtue of its beliefs the Left has a hard time getting pleasure out of most things society offers as entertainment.
Movies are too violent.
Sports are too commercial. Nice places to go are too full of yuppies. Everything is too classist, racist, and sexist. All this might be OK if the Left created alternative ways to recreate, celebrate, or just relax, but we don't. And because we are often on edge, critical, or being criticized, these absences take a great toll.