If one does X, then one receives Y.

Couldn't be simpler. If one mows the neighbor's lawn, the neighbor will reward you with some money. If you clean your room, you receive an extra Twinkie. Punch the bigger kid, get clobbered. Do the crime, do the time.  Quid Pro Quo. If X, then Y. Call it The Transactional Life. 

The difficulty in growing older or up or however you choose to look at the passage of time is that you start to see that this contract with the world, this Transactional Life, is not fair. Most of the time, the value you place on X is not in equal proportion to the perceived value of Y.