Theoretical physics/astrophysics for a few reasons... one, it's very interesting and abstract. Another, the maths you do just looks so fun. I often read stuff on Wikipedia and thinking about doing the maths almost makes me feel giddy. The degree is actually useful as well (my parents insist that I go to uni, even though what I initially wanted to do doesn't require one, so this works in my favor), and getting a Ph.D works for me. Chance to go overseas and it just adds another notch to my ego. The job is good too, if you make it: solid pay (not ridiculously high like some other jobs, but still very good given that I don't really care anyway), but what really sells it is the nature of the job. Doing research is a 9-5 job I can live with (generally I'm abhorrent to the notion) since it's, well, research, and you get paid to think about complicated things that you're interested in. Yeah, I'm sold. Theoretical is so I don't have to actually do anything (might be surprising, but I HATE doing anything experiments, and most hands-on things for that matter). I don't like the idea of proving things other people came up with anyway.
I also want to be an author, which has been a hobby of mine since I was in primary school. Why? Well, I like fantasy. Coming up with interesting characters, settings, stories and so on (with excruciating detail) has always been fun for me (and probably the only form in which I really like to express my creativity, I dislike visual arts and the like). It is an aspiration of mine to create the deepest fictional setting of any book or story around which I can base my written tales.
Oh, but there's more... I'm also planning on being a historian. This is one area of study that has always drawn my attention. I believe there's a lot to be learned from the annals of history, and I plan on researching and studying it by myself when I'm free.
There's philosophy too; I can definitely see myself a philosopher. By nature, I tend to think a lot and this naturally leads me into this field. Even if I don't formally become one, with a proper degree and so on, I have no doubt I would work with philosophy.
The two aforementioned areas are more of an overlap with my interests as an author, a non-fictional extension if you will. The reason why I'm not considering a BA/Ph.D in Arts is mainly because it kinda repulses me, the degree just sounds pretentious and useless. I like things to work with my taste as well, and being an "art student" just doesn't do it for me.
So there we go... why I want to be a theoretical physicist, author, philosopher and historian, courtesy of my new-found loneliness and misery.
tl;dr I want to be a theoretical physicist, author, philosopher and historian because those areas interest me.