Out of all the options presented, its the replayability that brings me back. Also, when I want to look for a change of pace, I try to actively build outposts and expand operation here and there across the map. I do this both in SP and MP. This keeps the game fresh for me because sometimes I have to change my environment to feel like things are progressing. I don't really have much fun sitting in giant fortresses and crafting bases for weeks at a time. And that's the key thing to me. When I get utterly bored of a map I make a new one, and start fresh and the challenges start anew. Once I get to about gamestage 150+ everything slows down considerably for me.
I think keeping things fresh is really what gets me to cycle on. Also, trying new scenarios REALLY digs me in. I @#$#$ love Romero mod in single player. The initial gamestage challenges make it damn near impossible for me to fully raid medium sized POI's from start to finish without having to drag the horde into the street and lining up your headshots. Large POI's are a damn nightmare, but it just... gets to me. The futility of survival. Also, loosing your backpack and toolbelt I find to be integral to actually feeling like there's a death penalty. If you never loose anything, the death penalty feels hollow.
That's when survival starts feeling like a reward, when you have SO much to loose.