For me it's about finding a situation that seems impossible at first, then eventually beating it by learning the enemy (both the actual enemies and the environment) and getting better myself.
That doesn't just include bosses, level design is a big part of it too, and it's what I enjoy (and sometimes detest) the most about souls games.
Edit: forgot the question I was answering, lol. Difficulty is an essential part of my experience, but more as a method and not the goal. It creates tension, keeps me focused, makes me notice things that I would have missed otherwise, and gives that great feeling of winning against the impossible.
I think that's Munich