1. Your relationship has devolved to the point where only an aversion to paperwork and inability to pay legal fees could possibly keep you together
  2. Because you're hoping to please an ancient deity who punishes anyone who has sex without getting his seal of approval first
  3. Sick of dog keeping you awake all night demanding that you add to the pack
  4. Dating isn't claustrophobic enough
  5. You're the former partner of a deceased treasure hunter and convinced that his widow has access to vast riches
  6. Gym membership expired
  7. The Supreme Court finally got its act together and now you have to put your money where your mouth is
  8. They do the dishes and you're like, I've gotta lock this down.
    Suggested by @ktenkely
  9. Green card
    Suggested by @p