I think a lot of it is the dopamine of getting to upgrade your character.

Also, I have observed that people LOVE getting everything together to get kitted out for a mission. If there’s some special equipment they’re going to need to go into the temple, and they’re trying to think what they would need once they get there and running around town putting it all together, they just get super excited and it gets them amped up for the adventure. It is fun in my experience, although yes YMMV.