With me, the game has to stay challenging in order for me to stay interested. I must feel like the game can always take my life at any given moment. I will not enjoy the game if I'm one-shotting everything.
Formerly, I used to start on Novice and adjust the difficulty when things were getting too easy. Naturally, when playing Skyrim your character seems to advance faster than the difficulty can catch up, if you're only focusing on your offensive and defensive capabilities.
Right now, I am playing on Legendary and going VERY slow. I'm only level 3 so I'm taking every precaution necessary in order to be successful at the game. Because the difficulty allows a change from 1/4 damage from you while the enemy is bringing 3x damage as normal, it is enough to keep me challenged to go forever.
I've learned through my experiences though, not to rush through the quests. It's better to take your time and stop by every dungeon before venturing forward. Once all of the quests are done, there's really nothing to do, so stopping to complete the dungeons and enjoy the scenery helps me play longer.