A friend of mine (who happens to be a widower) shared this on Facebook the other day and I think it's incredible. For a full and wonderfully in-depth explanation by the originator of the post, read this article.
The gist of it is this. Grief is like a box with a pain-inducing "trigger button." Right after the event happens, there's a big ball in that box that smashes against the trigger, keeping the pain level high. As you begin to heal, the ball gets smaller, leaving more room and taking the pressure off the button sometimes. There will still be times when the ball bounces around and hits the button - less frequently as time goes by and the ball continues to shrink.
It's important to note that different things can bounce that ball around for different people, and not everyone has the same sized ball, even if they experienced the same grief event.