It’s a combination of things. I like leveling, I like collecting things (race/class combos), and I enjoy trying to optimize how I level, etc. Those are the positive reasons.
The negative reasons are… I hate time gating. I tend to play LONG sessions but not every day, so I run out of pertinent stuff to do on my primary characters. Once I’ve done the dailies / world quests that matter and done my weekly chores on my primary characters, I’m left with hours of game time and i wanna play WoW.