Resources and survival.
For example, platinum is $1000/oz on earth, but you can find city-sized chunks of the stuff in the asteroid belt. It could become as widely used as aluminium if we mined there. This one is tricky though as capitalism means we'd only do it if we can profit.
The traditional argument is that if setting up this operation takes generations, then it will require some very rich and forward thinking corporate investors to fund it. Not many individuals would fund it, because they would die before seeing their profits.

Long term, we need multiple colonies to ensure survival, e.g. if a dinosaur-killer rams earth again, mars is a specimen bank. There's no capitalist incentive to pursue that one though.

This also sets us up for a corporate distopian future, where corporate colonies are isolated enough from earth to basically be their own dictatorship nations... At that point, there's also the incentive of being able to get away with whatever you want, such as slave labour...

It's going to be the eccentric billionaires like Elon Musk who make this into a reality. No government will take it upon themselves to save humanity from a planet-scale disaster, but an individual who already has wealth more than some nations can afford to be forward thinking.

FWIW, multiple companies are already prospecting in space today, with plans to actually start mining in as soon as 5 years. This is already reality, not sci-fi :o https://en.wikipedia.org/wiki/Asteroid_mining

We already mine the bottom of the ocean with the use of completely automated robotic cities, in an environment that is actually harder to operate in than space :wink: Mining itself isn't that challenging, as far as I know, the really hard problem is how to land a large amount of your bounty back on earth safely.

There's also adventure. If we send a generation-ship to another star system, there would be enough people who would volunteer to crew it out of a sense of adventure, heroism, duty, etc... It's a return to how things were before we'd charted the entire globe. There's people on this forum who signed up for a 1-way trip to mars...

And hand in hand with that is tourism. We're going to have 99% of people leading shitty lives, and 1% of 1% of people rich enough to go to the moon for fun. There will be an industry to support those desires.

In the near term, the Mars vs Venus debate is pretty simple: We have the technology and engineering ability to build cities on Mars, we don't have the technology or engineering ability to do so on Venus. (Nothing says you have to be able to walk around on the surface of Mars without a space suit for humans to live and expand civilization. Living in self-contained subterranean settlement networks makes a whole lot more sense.)

The issue of resources isn't much of a good reason to colonize. I can't imagine any sane business relying on dudes in space suits floating out there cutting up asteroids and lugging them back to earth for the simple reason of they are terrible choices for the job. Seriously, why would a live human need to be out there when a robot could do the job far easier - For little more than the mass of the tools involved, the job gets done. In space, delta-v and payload mass are king. Humans need too many supplies to keep them alive, weigh too much in and of themselves, and need too big of a ship to be worth it compared to sending a robot out, having it mine and refine the target payload, then sit there floating around till it is in the perfect location to do a Newton's Law-Burn. - Payload is flung towards earth with a catapult while at the same time serving as the reaction mass to fling the mine bot on toward the next target.

As for population issues: Well, modern science and living is taking care of that nicely. Population growth isn't exactly booming in the developed world after all, and earth isn't exactly short on the majority of elements that we use. Oddly enough we tend to use elements that are actually pretty abundant on the planet, and the key issues of demand aren't "We don't have enough of this!", but rather "We don't have capacity to process this fast enough!".

As we advance towards a resource conscious society with growing automation, most of the dooms day sky is falling resource problems kind of fade away. Some estimates suggest that we already have enough gold refined to give every human nearly half a kg of it. That's a lot of electronics... There are only so many sky scrapers that we can build, or want to build. Get enough families in safe modern lives with good health care, and the long term worry isn't going to be "Where will we put everyone", but instead become "How can we convince young people to have more than 0-1 kids?"

Why humans would go to another planet, settle, build cities, and raise families is not going to be as a way to "Get more land" or "get more resources", it will be "Because we can". And hopefully humanity eventually builds the infrastructure needed to continue on beyond our own solar system to grow and expand and a species on a shared believe of "Because we must". Growing, exploring, expanding is human nature.

Yup, population growth slows down or even inverses in the developed world. I don't think that our lifespan will be increased or we will even life forever either (most people just couldn't do anything with their time anyways), so population won't be such an issue.
I think a good thing would be to get the most energy-consuming industries off the planet, so all the polluting/demanding factories with all their support facilities and logistics would get away decreasing the energy demand and pollution.
Explanations of the Fermi paradox say that eventually an intelligent lifeform will either kill itself before colonization or learns that it's too expensive and doesn't worth it. Maybe this is a reason why we haven't found aliens yet.

I find the survival goal to be interesting, philosophically.

So, earth could get wiped out by a space rock any day with little notice, so we really should have some off-world colonies... but then what? Eventually the sun is going to kill the entire system, so we need colonies in interstellar space / in other solar systems... but then what? Eventually expansion will isolate our galaxy from all others, so we need FTL transport to keep colonizing galaxies... but then what? Eventually every star burns out and the great heat death happens (or the big crunch, or whatever the other end of time is...) and it's inevitable that we can't just keep running forever. Eventually survival is futile.

Individually we all die and eventually have to come to terms with that, and if we evolve into a intergalactic society that can survive for millennia, then it doesn't really change that -- eventually as a species we have to eventually learn to accept extinction. That's an interesting process for an individual - having to accept that fate - and it's a well known process by now... but no one knows what the process will be like for an entire sentient species!

Sure, natural population growth is on the decline in regions in the economic north. The remaining 90% of the planet remains in a Mathusian state of population explosion, and most of those places are dominated by cultures less friendly to what yer typical middle-class westernized white male would call 'progress.' Extend the argument that your little walled community of privileged kids isn't having children and the conclusion is not that the Earth's population will level off and then decline, but rather that the insatiable demands of the unwashed masses will overwhelm and we'll have a global collapse into a dark age with consequent mass starvation, rampant disease and pestilence, and war.

Either we need to expand offplanet for more, safer, living room for the rich elite, or else as a place to send the ignorant poor to keep them at arm's length.

Is that the right reason to expand off the Earth? No. That's just a practical one. The main reason is because it's there.