Why do we keep doing things that we know aren't worth it?
I eat fast-food probably once a quarter (doesn't matter what, I have no particular loyalty to McD, BK, or Wendy's). It takes about 3-months to forget that I regret fast-food more than I enjoy it.
I'm pretty sure fast food isn't the only activity with this characteristic. Bad Reality TV seems pretty good while you're watching it, but after you've blown 2 hours, you start to wonder why. Meaningless sex is a definite candidate for the “not worth it” distinction.
Be sure everything you do comes with a conviction that guarantees you won't look back later and say “not worth it”.