I find that the game distills into objectives from which different archetypes emerge. Most players sort of define themselves within one and their purpose for playing the game emerges as a consequence.
There are two primary archetypes:
The Completionist. Every quest, every shirt, every achievement, every raid, every difficulty, and everything else. It’s all on the to-do list of tasks that has to be done for the penultimate goal of having done everything and acquired all there is. The God-complex and the desire for 100%.
The Challenger. The goal is to beat the challenges within the game. So KSM, AOTC, Glad, and other construed feats of difficulty that exist. Every Season is a new challenge to beat.
These two archetypes differ in the sense that the completionist doesn’t really care when and how they beat a certain difficulty, they just want it completed alongside everything else. The challenger deliberately seeks to beat a difficulty because it is difficult and wants the challenge from that and doesn’t care much for anything else.
So that’s people on the forum with 40k achievement points versus RWF contenders.
And then the no-lifer is of course the person who tries to do both. Very few are able to sustain that approach to the game.
From these two archetypes there are several sub-types that players tend to settle on, because they’re more manageable.
The Collector. The goal is to collect something or anything that can be piled in a collection. So mounts, pets, transmogs, toys, and so on. It’s the never-ending carrot-on-a-stick chase. You settle on some McGuffin you want to collect, and then you pursue 100% of that thing. It’s a sub-type of The Completionist.
The PvPer. A sub-type of The Challenger, but limited to one aspect of the game – PvP, and sometimes limited even further to just Arena or BGs.
The Raider. Also a sub-type of The Challenger with a focus on raiding and progressing through and beating the difficulties, contenting either with AOTC or striving for Cutting Edge.
The Alt-o-Holic. A spawn of The Completionist, but with a focus on the gameplay, i.e. having done all the races, classes, factions, zones, or quests, or having everything at max level.
And there’s more, right? RPers, Pet Battlers, M+ers, Open World Solo players, Auction House players, and so on.
And every type seems to revolve around a pursuit of completionism or overcoming a challenge in the game, either broadly and loosely defined, or very particularly and concrete.
And everything Blizzard stuffs into the game is basically food for these arche-types. More stuff to collect, more stuff to do, more stuff to experience, more stuff to beat.
And the “I’m bored”-feeling tends to come when the pursuit of completionism or challenge feels meaningless. Why collect more mounts? Why do higher keys?
Once that WoW version of an existential crisis hits, then you’re done. Then it’s time to take a break or quit entirely. It’s a break if the urge to collect mounts or progress M+ keys re-emerges. If that motivation doesn’t reappear, then you’re quitting.