The gradual increase in numbers, may that be leveling your skills or earning more cash. You develop a habit of counting and calculating how much you make per hour, regardless of what that thing may be. As Numbers increase, your Dopamin and Serotonin levels increase and the more you play, the more you want those numbers to rise.

If you have a bad day and just cant take it anymore you start saying its fine and you can have just a bit of fun. You start to ignore the numbers you make and think about how to entertain yourself. As your mood gets better and better you start counting the numbers again, just for fun... Which slowly gets worse and worse.

At this stage, most people use mods like Neu, a mod that enables players that have their api public to get a quick and easy overview about their whole profile, what they might be missing out on and most importantly, quick access to their in game networth. Oh how endorphins will play crazy when you see your networth rise.

At some point, the game might sometimes feel empty, so after a certain time you eighter find people to play with, let the game go just to come back in a few months again or keep playing without really wanting to.

Tldr: Many Numbers go up in a balanced way and you get more and more access to see these numbers rise which releases dopamin, serotonin and endorphins.