@Jamie John I've been on a similar run lately. In the last 18 months I've finished Hollow Knight, Bloodborne, Sekiro, and Cuphead. Currently on a first proper run of Dark Souls too. I definitely hear what you're saying. I do wonder at times if I'm just after that rush of beating a really hard boss. But I actually think it's more than that, more than just a release from frustration. Each boss is a difficult puzzle that needs to be solved. And even when a viable strategy has been found, it needs to be executed perfectly. I think Cuphead excels in this gameplay.
I totally see why people would bounce off Cuphead though. It is brutally hard at times. I just really loved the style and animation, and seeing the next wonderful creation was a big enough reward to keep going.