The reason you should do what you love is because you won’t work hard at it if you don’t love it. And hard work is, in fact, more important in success than raw talent. The guys who wrote Freakonomics also write a column in the New York Times magazine, and this topic is the focus of their most recent column:

“When it comes to choosing a life path, you should do what you love — because if you don’t love it, you are unlikely to work hard enough to get very good. Most people naturally don’t like to do things they aren’t “good” at. So they often give up, telling themselves they simply don’t possess the talent for math or skiing or the violin. But what they really lack is the desire to be good and to undertake the deliberate practice that would make them better.”

Of course, the difficult part is to figure out what you love. Maybe a better standard, though, is to figure out what you love to practice. That seems to eliminate a lot more possibilities right off the bat.