Sep 27, 2015, 9:04 PM

@Apu said:

I think that to a lot of people getting XP is gratifying although I can't say why. Maybe they see it as a reward for all the time they spend on a game or something.

People are literally satisfied by watching a number go up.

http://progressquest.com/

and

http://orteil.dashnet.org/cookieclicker/

Have both proven this beyond all reasonable doubt.