Every season I stop when I unlock the last reward of the battlepass. S4 and S5 I reached that when my characters were 96.
Only 4 levels to lvl 100, but why would I do that? I don’t care about doing the same thing on the nightmare dungeons only with higher numbers…
In MMO’s you can grind for things that last “forever” like gold/mounts etc, the equivalent in D4 would be items I guess, but get changed/buffed so much that is pointless to try to hunt them
So what keeps you playing?
Like all other games, I play because I’m having fun. When I stop having fun, I play other games. For D4, it varies season to season. For instance, I basically skipped S3, but in S4 I played all but 3-4 weeks.
I am only level 71 right now on WT4. I will most likely stop when I have all of the Mothers Blessing, the Season Journey completed and all the fee stuff from the Battle Pass. Maybe go to level 100. I want to take some time off before S6 and the VoH. I will read the crying on the forum. 
I do not care about PvP the cheater boards or the pit.
I’m having fun playing something others probably are not, a sorcerer but with only a single skill on my bar. Seems to be working great. lol. Tier 4 Hell cleared. Not bad for a solo player.
The purple dropping from the sky is an addiction in itself. I must see more
Eternal only here (for now at least,) so I am already a player who plays a character longer than many, and honestly I am wondering that at the moment as well. Every game ends eventually, but even so–
The loot reborn update brought me back after a couple months of inactivity because it finally made gear something I could more or less procedurally upgrade, not just do the same content a million times hoping something with completely random affixes that was actually somewhat better than what I had dropped. (I hit level 100 on my main during the launch to season 1 period and played consistently for quite some time, and I wasn’t really getting better gear after awhile.)
Finally having aspects out of the inventory and being able to trash the dozens of items I had that would be upgrades if something else good dropped on another slot that would take care of what I lost equipping the first piece (gearing was very much about having to get complementary pieces since affixes were basically COMPLETELY random) gave me the room to start a second character.
Now though - I don’t know. I’d like to start a third, but two back-to-back updates with massive itemization changes as well as an expansion two months away that might have higher level loot have soured me on the game, as constant purposeful refarming sucks. The infernal horde mode gives garbage rewards solo and I don’t like to play in groups except for the one friend I have who still plays this, and she only plays about once a week, and since that makes it not an alternative to pit (I only need Neathiron and I can do pits 80-90 but I think tier 7 horde is too difficult at the moment for me, and tier 6 doesn’t drop Neathiron) I’m basically still stuck on pits.
So start a third character? Probably not, since I may have to re-farm everything in a couple months anyway and I won’t be able to interact with the horde in a way that actually benefits that character’s growth.
You just releveled again but you don’t like doing the same thing again? Makes sense lol
I swear some people forget that video games are for fun - nothing more and nothing less for 99.9% of us.
Like you, how long I’ve played D4 has depended on Season. Season 0,2, and 4 were essentially the entire season. Season 1 and 3 I didn’t play that long.
Combat is fun when you have a build you enjoy in my opinion and the game doesn’t require crazy focus so you can relax while playing. Makes D4 a nice “play and chill” game unless you really want to push hard.
I have games for “focus and challenge” and that isn’t D4. It is just like D2 before it. I keep coming back because I can relax,n play, and grind at items in a fun manner to me.
I guess people keep playing or stop playing for whatever reason they choose. Who am I to judge? As long as I am having fun, I play. I don’t understand how that is not the ONLY answer to the question.