Math is fun and interesting and entertaining. This is not why I do math. When you know something is true in math, you know it's truth is unassailable. This is also not why I do math. I have always been good at math. This is not why I do math. Say I have a question I've been thinking about for weeks. Something about it intrigues me so I work at it, and then I struggle with it, and then I rage and rail against it. I pour my heart and blood and sweat into it. I turn around and I haven't slept, haven't eaten and haven't seen my friends in days. Worst of all I've made no progress. At that moment, that moment when I've fallen into the deep yawning chasm of depression, that moment when I know I've failed, that moment when I finally realize how useless and stupid I truly am, something clicks.The world opens up and I'm on top of it. I'm unstoppable. I'll go for a walk. And while I walk I realize that actually, the answer I came up with has some pretty interesting implications to something that I heard in a lecture last month. But no it's more complicated than that, so I start to write down some questions. I don't love math, but this is why I do math. (Mayer-Vietoris)
Three reasons, in order of importance: It's fun for me, I have some skill in it, and others are willing to pay me to do it. Any job that satisfies all of the above is a dream job. Stick with it as long as you can and as long as all of the above are still true. Loving your job makes loving life a hell of a lot easier. If any of the above were missing, I don't think I'd be able to keep motivation enough to stay in the academic world. I'd probably look for a job in the private sector, with a resume that reflected my core asset: I can solve abstract problems. This is perhaps a fourth reason I love math: It's a highly useful skill. Even if I have some sort of crisis which forces me out of grad school prior to earning my PhD, I feel that I wouldn't have much trouble finding a job that I'd enjoy.