Check out this article, by a computer science professor at Georgetown named Cal Newport, which is about how the idea of "passion" does and doesn't fit into career choice and exploration. In essence, he says that passion should follow rather than lead a career.
My take on this: People think that the order of satisfaction is (1) cause (2) people and (3) function, but it's really (1) function, (2) people and (3) cause. If you are working for a cause, but don't actually like what you do each day, you are going to end up bitter and unfulfilled. Whereas if you actually like doing the activities that comprise your work — whatever they are — you are more likely to be happy and contribute something useful to the world.
I didn't really know this when I was younger and making certain key decisions. But it's true. Luckily, I have learned from experience and now apply this thinking to my choices.