But where's the fun in that? Seriously; even if we start treating our environment better (something I'm all for, btw) and every country on earth puts severe population controls in place (something I'm vehemently against, personally), we'll eventually run out of space and resources (though probably not in that order). It may not be for another thousand years or more, but it is a fact that the resources on this planet are finite; they won't last forever. But that all aside, why shouldn't we spread ourselves to places other than Earth? Why should be limit ourselves to a single planet when we could have so much more? When we could be so much more.
The way I see it, there are only two things that stand in the way if such a future: the belief that we first have to perfect life and society on our planet before we can even consider going anywhere else, and short-sighted people in positions of power.
The first I mention because the most common argument I hear against the space program is "There are so many horrible things happening here on Earth. How can you justify spending resources on something like this?", or something along those lines. The answer to this is quite simple. To borrow from a philosophy that I've seen in several places, there are two ways to improve the universe: by decreasing suck and by increasing awesome. I believe that these two goals are both very important, but all too often people (and governments especially) focus exclusively on decreasing suck...and while that is a great and admirable goal, there is more to life than that. Case in point...this entire website. Case modding is one example that is almost purely increasing awesome. There are a few exceptions, such as cases being auctioned off and the proceeds going to charity, but for the most part there is not much suck being fixed by casemodding. We do it almost exclusively just to make cool things. And that is, imo, just awesome.
The second thing, short-sighted people in positions of power, is a problem because until the last ~20 years or so, all space travel R&D has been funded almost entirely by governments. And almost all of the people in control of those programs' funding are, by necessity, relatively short-sighted people. The types of programs that would really move us forward in the endeavor to expand to places other than Earth are programs that would likely take decades to reach completion...and possibly even longer before major returns are produced....and that is not something that it is easy or likely for someone worried about getting re-elected to support because it is also not the type of program that the majority of the voting population would support. This is one reason why I do not think it is entirely a bad thing that space research is moving more and more into the private sector.