I've said it before and I'll say it again: the only good reason for doing a PhD in any subject P is that you're so fascinated by P that you find youself saying:

I've said it before and I'll say it again: the only good reason for doing a PhD in any subject P is that you're so fascinated by P that you find youself saying: "Holy shit, you mean somebody will actually let me sit around and just do research on P for a few years? And they'll even pay me a little bit of money while I do it? What a sweet deal!"