And I don’t mean just that lost happy moment with his now-dead wife that the hero is trying to avenge, I mean something that he can possibly return to after the movie is done. Something that makes us say, “If he gets out of this, he could have a pretty great life.” These moments throw everything else into relief. The brighter the bright spot, the darker everything else is in contrast.
And this should be true of every piece of writing. Are you writing something mournful and poetic? It should have one big, crude laugh. Writing a romantic comedy? It should have one brief glimpse of intractable tragedy.
When he was developing his photographs in the darkroom, Ansel Adams made sure that every print, whether the negative was shot during nighttime or daytime, wound up with precisely one spot of true white and one spot of true black. Those two points define the range of values and throw everything else into relief. Writers should follow the same rule.