The Happiness Algorithm

The algorithm knows you better than you know yourself. And it uses that knowledge for one singular, driving purpose: to sell you more, more, more. Until a rogue programmer rewrites the algorithm to show you not what you're most likely to buy, but what will make you happiest. All over the world, people are smiling on the street. Quitting their jobs. Finding love. The world is a happier place now, and what could be wrong with that? But some people's desires are darker than others. This short story is 7200 words long.
It is also available in Digital Soul, a science fiction short story collection.