system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it easier to dwell on upsetting moments rather than happy moments?
gpt
Memories get bound more strongly the stronger the emotion that accompanies that memory.

Oftentimes, happiness is simply the absence of negative emotions.  Yes, sometimes you feel ecstatically, exuberantly joyous. But most happy memories are the moments where nothing in particular is stressing you out, and you are calm and peaceful.

Compare that to when you feel very angry or scared.  Memories are not only stored cognitively with its sensory perceptions (sight, sound, taste, touch, etc...) but are also stored in your midbrain in emotions.  The emotions get stored in one place, the sensory/visual memories in another (like in the hippocampus but not only there).

When you have a negative memory of strong emotion, your emotional brain (midbrain) remembers the emotion.  It makes it easier to conjure up that sensory memory, especially whenever that particular emotion surfaces for any reason again.  

Happy moments are usually the absence of negative feelings, and very very happy memories do get remembered vividly, but then again, often produce nostalgia or even sadness or grief, as the memory ended.