Out, I've come and gone to many forums in my day, and I've rarely felt ANY need to justify my leaving. From time to time I'll meet a few friends and say good bye, but that's about it.
When you get bored of it, simply STOP POSTING. There's no need for psychoanalysis. Just stop it.
It's sort of like when your body tells you that you've had enough sugar or sodium. You don't have to consider *why* you no longer crave such foods. You simply have to stop eating them to get your system back on track.