I'd like to second an event.  That's basically what keeps life interesting.  Might be a job opportunity that requires you both to move.  Might be a job opportunity that means you'll have to be a long distance relationship for awhile.  Might be a big fight that has you living separately for a bit.  Might be a death in the family.  Might be one of you become an aunt or uncle.  Could have an old flame showing up and one partner is worried the other might cheat on him/her with that old flame.

There's always stuff to make you reassess your decisions.