[derefr]

This is a case where money and happiness aren't really correlated. Remember, people get addicted to variable reward schedules, and get bored of fixed reward schedules. Want someone to be more excited to go to work? Flip a coin to determine whether they get paid each day!

[sireat]

This is also why games put random drops in your grindable content.

And why despite the satiric premise behind http://progressquest.com/ it is weirdly addicting.