I think the only way might be to add him temporarily to your household, because I do not know of any other way than checking moodlets. If they are there at all when you include the sim, they might tell you what happened. But - when "removing from household" that sim will not return to previous household but probably start a new household or not be in a world at all. Sounds like a sim you'd like to keep, so you might need to reaarange stuff in "manage households"...
Keep in mind that the game might add random stuff to NPCs, so there might be no backstory at all for the sadness.
I never tried checking details like this myself. Will be interesting seeing what you find, if anything :)