Why can't I be an astronaut?

Okay, so if you ask a little kid what they want to be when they grow up, they may well say something like policeman, fireman or astronaut. However, if you ask someone who is in the process of applying to university what kind of job they want, you wouldn't expect astronaut as a response, probably. Which means I may have a slightly awkward time during any interviews (To be honest, I'm also considering being a firefighter. But that's a different post).

A lot of people treat this with an understandable amount of skeptism. Or they do a little 'Aw!' noise which is actually worse. Because, whilst these people all think I want to be an astronaut because I'm immature (which is kinda accurate) or because I don't actually have any life plans, I have put a surprisingly large amount of thought into this:
Starting of with the why. I want to be an astronaut for more reason than the obvious 'It's pretty much the coolest job in (and out) of the world'. I love science a lot. It's something that I already spend most of my school life doing and want to keep doing more of. Science is just really awesome because we can take something we can see and come up with an idea of why it is acting this way, which is then tested by doing experiments. Space is a very handy place for doing experiments. It's such a different enviroment to Earth that loads of different things can be done and learnt. And I would love to be able to do this kind of thing for months at a time, hence why I want to be an astronaut (but I will admit that the incredible views and fun microgravity are nice perks).
Even training how to become an astronaut seems like fun. It's not just about getting used to the completely different environment, which still seems like an important part, it's also about the technical and scientific side of it (because there's no point in going to space and then not knowing how to operate any of the equipment - it's not like you can just ask the tech guys to come round and fix it!). Considering that I love learning new science things (I've never learnt about engineering before so that would awesome), it would be perfect!

I've gotten so far as to consider the potential drawbacks and obstacles to overcome. Things that might help me in becoming an astronaut:
 - Be really physically fit so I can cope with the physical strain of being an astronaut
 - Learn Russian (The ESA has a training base in Russia apparently so that makes sense)
 - Have an interest in American/Japanese/Russian culture (So everyone gets on nicely in the International Space Station :3)
 - Learn how to Scuba Dive (All astronauts doing EVAs (Extravechuilar Activities) need a basic dive certificate.So, you know, if I start now, I may well be well qualified if they reopen recruitment!

Yes, I agree it's still a little outlandish. I am still planning a non-space based career as a back-up option. But it's good to dream I suppose. Because, even if I don't become an astronaut, doing all this research and learn about other cultures and new scientific advances - it's certainly not wasted regardless of what I do.

P.S. If anyone out there (in Europe that is) is equally interested, look on the ESA website! (European Space Agency) http://www.esa.int/export/esaCP/index.html It's really cool and I spent hours of 'homework time' looking on there!
P.P.S. Not all the information I've written above is necessarily accurate. I am very good at making errors. So, go now! Look at the ESA site! It's a lot more reliable than the ramblings of a 17 year old :P