This is a hot-button topic. Beware.
Attempts to discuss the prospects of human exploration and inhabitation of the cosmos on the internet tend to attract a certain type of participant. If you've been following the comment threads here you probably recognize them ...
For starters, they're overwhelmingly white male Americans (plus a handful of Brits and Canadians). Politically they're right-of-centre (by American standards), and libertarian-leaning. They are enthusiastic proponents of space colonization, but will boost any other technological or scientific work oriented in an upward direction (as long as it's carried out by people who look like them: they're somewhat less gung-ho about the former Soviet, and now the Chinese, space programs).
There is an ideology that they are attached to; it's the ideology of westward frontier expansion, the Myth of the West, the westward expansion of the United States between 1804 (the start of the Lewis and Clark expedition) and 1880 (the closing of the American western frontier). Leaving aside the matter of the dispossession and murder of the indigenous peoples, I tend to feel some sympathy for the grandchildren of this legend: it's a potent metaphor for freedom from social constraint combined with the opportunity to strike it rich by the sweat of one's brow, and they've grown up in the shadow of this legend in a progressively more regulated and complex society.
My problem, however, is that there is no equivalence between outer space and the American west.
Humans are a climax organism that is fundamentally dependent on a couple of key ecosystems. There's the one we carry around in our guts — about a kilogram of bacteria and fungi, for a typical adult — without which we can't even digest most of our food. And there's the ecosystem we live in. (Or ecosystems. Because of our unique horizontally-transferable tool culture we can adapt to existence in terrestrial ecosystems other than the one our ancestors coevolved with. But there are limits; we don't thrive in Antarctica, or at the bottom of the ocean trenches.) We're also somewhat dependent on our extraordinary extended phenotype, from flint hand-axes to Space Shuttles. Maintaining that phenotype is a large-scale operation supported by a penumbra of extended cultural activities that maintain the ability to maintain the phenotype — primary school teachers, for example, don't bend metal but are absolutely vital to the activity of engineering insofar as you've got to start educating your next generation of engineers somewhere. Hence some earlier postings on this blog.
Basically, it's not clear how large a system you need to support human civilization. We don't know how to build biospheres from scratch yet, and indeed there's worryingly little research being done on the topic (which may become a screamingly important priority in another half century, if the most pessimistic climate change projections are accurate). We can make a rough back-of-the-envelope guess at the size of human population it takes — given abundant raw materials and a favourable biosphere — to maintain a technological civilization; it's many orders of magnitude larger than the proponents of Heinlein's nostrum that "specialization is for insects" may be comfortable with.
There may be possible technological solutions to both problems that don't require the combined lifelong effort of millions of humans. We don't have (a) strong artificial intelligence, (b) self-replicating machines that can work from raw materials extracted from their natural environment, (c) "magic wand" space propulsion technologies (which may themselves be Fermi paradox solutions insofar as their existence implies either flaws in our current understanding of physics or drastically efficient and thereby destructive energy sources), or (d) the ability to re-engineer ourselves. If any one (or more) of these are achievable, then all bets against space colonization are off.
But. But. But.
The west was inhabitable; it supported a healthy set of interlocking ecosystems in most of which a lone human being could find food and sustenance. (There were exceptions.) It already supported a human population. The colonists were equipped with adequate technology and were crossing distances that were, at a pinch, amenable to shanks' mare and a walking stick.
These conditions do not apply in space. You don't get to breathe the air on Mars. You don't get to harvest wheat on Venus. You don't get to walk home from an asteroid colony with 5km/sec of velocity relative to low Earth orbit. You don't get to visit any of these places, even on a "plant the flag and pick up some rocks" visitor's day pass basis, without a massive organized effort to provide an environment that can keep the canned monkeys from Earth warm and breathing.
I postulate that the organization required for such exploration is utterly anathema to the ideology of the space cadets, because the political roots of the space colonization movement in the United States rise from taproots of nostalgia for the open frontier that give rise to a false consciousness of the problem of space colonization. In particular, the fetishization of autonomy, self-reliance, and progress through mechanical engineering — echoing the desire to escape the suffocating social conditions back east by simply running away — utterly undermine the program itself and are incompatible with life in a space colony (which is likely to be at a minimum somewhat more constrained than life in one of the more bureaucratically obsessive-compulsive European social democracies, and at worst will tend towards the state of North Korea in Space).
In other words: space colonization is implicitly incompatible with both libertarian ideology and the myth of the American frontier.