>It seems a bit simplistic when stated that way, but I think there is something to it.

I think this is pretty uncontroversial and you can observe it everywhere. Even in music, if you want the beat to hit harder, take it away for a short period, and when you bring it back it will feel like it hits harder and with more energy even though it's exactly the same volume as it was before.

Though it doesn't really explain how some people are continuously more or less happy. If the brain only cared about change, you could only ever be an average amount happy. Clearly something about continuous discontent and negativity still impacts you even if it might dull.