Simple, by way of metaphor:
Today I redownloaded the demo of StarCraft, a game I've played and loved in the past. Completing the final mission, meticulously exterminating Zerg with ranked Siege Tanks (the only sure way to have zero friendly casualties on lower levels), it occurs to me:
I'm wasting my time.
There is stimulus in winning a game, but it's artificial stimulus, without use in the real world. That's why I stopped (now I remember - like eating McFood, occaisionally you forget why you don't eat it, so you eat it again...)