Depends.
The long answer, or the short answer?
Long: The human body craves stuff and craving causes pain if its needs are not met. The bigger the craving, the bigger the fulfillment when the deed is done. Constantly rewarding yourself diminishes this function, leading to need of increased stimulus to maintain the state of clarity at base value. There are various ways to prolong the effect, through pharmacological and behavioral practices (and brain-machine interfacing, but let's not go there).
TL;DR your body is greedy and it's your choice how you reward it, lest you become slave to your desires.
Short: Weed.