Before human beings developed external memories (photographs and now digital records of everything we do), trauma and negative memories were supposed to slowly fade away, and they did. Only mildly emotional residues remained. Our psychological itch to keep experiencing sights and sounds associated with a thing or person lost could not be scratched no matter how tempted we were.
Now we can. Not only that, we consider it some sort of obligation to carry the burden. We scratch the wound just as it starts to heal and it's like experiencing the loss over and over again.
or may be that's just my experience. I used to be in a permanent state of melancholy because I always reminisced about past losses, even when the present was good. I don't do that anymore. I've a rule, in fact: never replay a negative experience more than once unless you have new data to re-evaluate it in a different light.
Remembering good things (and apparently the past tends to be seen in positive light, regardless of whether we felt happy at the moment) causes the mismatch between our memories and the reality. Possibly because we're very social species, a deceased person causes the worst pain.