Classic-Wrath were the glory days only getting better and better for me. Cata was kinda the decay. MoP I think was the first time I felt like I didn't have a reason to log in and play any more aside from just raiding. I didn't even bother with WoD, and the only reason I played in Legion was to attain some arbitrary goal of getting every cutting edge achievement in one expansion... minus Helya cuz some part of me just didn't care about that boss. Turned down the opportunity to clear it even strangely enough, but I think that was something to do with my ex at the time if I remember correctly. She hated me playing WoW.
I dunno though. from Classic-Wrath I felt like there was always some reason to log in and do SOMETHING. Learn something, explore something, meet relevant people. Maybe the game's older reward structure resonated with me more? Farm for X thing and then you're done with it forever. I do kinda miss those grinds. Argent Dawn rep for BiS shoulder enchants. Sons of Hodir for BiS whatever enchants. Stuff like that makes the game feel more rewarding to me than farming randomly hoping I get titanforge procs.