This. Mammals are (generally) K-selected species, meaning they invest heavily in raising their young. In the absence of natural pressures, mammals reproduce like crazy until they bump up against the environment's carrying capacity. Humans are not unique at all in our tendency to expand! It's just that we have opposable thumbs and language and tools to help us boost the carrying capacity.

Yeah 'instincts' are in real world just enough external pressure and death to keep the equilibrium going, whatever grisly happens behind the curtains.

Hunters hunt as much as they can. Wolves regularly kill 10 or 20 sheep while eating one if they get the chance. Foxes do similar stuff with chickens. Nature is brutal and without empathy.

They don't develop this equilibrium "instinctively" (something exclusively inside them) but they do "naturally" (helped by the environment). Now the reindeer weren't really in their natural environment, they were put in very constrained, special conditions, with little flexibility, little time to adapt, and no ability to shape that environment. The environment forced them to adapt and lower the numbers, and eventually wiped them out with what was also probably a fluke. They were still 50% more individuals than when they arrived but no viable reproduction path ahead.

This was an extreme example. Put humans on this type of island and you'll probably end up with them dying out just the same, despite our tendence to radically change the environment to survive. After all that's why the reindeer were there, so humans can survive absent a constant lifeline from civilization.

Humans, and viruses to a degree, are much better at shaping their environment and adapting faster to what's thrown at them to compensate. The instinct is to change whatever possible of the surroundings to survive and thrive.

> Why is this a desirable goal?

It doesn't have to be a desirable goal to everyone.

> The only driver that I can really comprehend is the desire for freedom and autonomy in less populated spaces.

You got one of the big ones. But not the only one. Other is survival. Here on Earth we are all one bad infection outbreak away from ending human society as we know it. We have all of our eggs in one basket. Even if we would have a stable foothold on the moon and mars we would still be vulnerable to gamma-ray bursts and crazy despots with nuclear armed missiles.

> We'll recreate the same problems we have here everywhere we go.

We do. There are still benefits to the people who are "taming the frontier". And that is enough for it to happen. We also see that even though human condition follows us different places have a different feel to them. Some places we got some things better while others worse.

> Compared to anywhere else we know about, Earth is an extremely unique utopia.

To a certain extent. We can adapt the environments to us. And we can adapt ourselves to new environments.

When I move to the arctic I leave my parasol at home and buy a coat. When I move to a gas giant I need to rethink more of my biology. Imagine if some of us can become a buoyant sail with manipulating appendages who feels as much home in the red dot of jupiter as a homid feels home on a dewy meadow. If we could I would for sure give it a go for a few hundred years, then come back and write a book about how it was.

The fact that this is not easy is part of the lure of it.

Spread the risk and reduce the probability of extinction.

We know for a fact that earth is doomed, on top of our own continuous efforts to kill ourselves off. No not recent climate change type of doomed, but the evolution of our sun is continuously pushing the habitable zone outwards. We might be able to deal with that particular annoyance by hiding underground when it becomes an emergency in half a billion years or so, but our utopia won't be as utopic anymore.

Eventually however, the sun will balloon to a red giant at which point we better have a plan in place other than staying on this planet.

I don't really think about this much, but your comment made me wonder:

If we do find another earth-like planet within travel distance (impossible afaik but let's suspend disbelief for a moment), how do we determine whether it's worth colonising? And how to we measure it?

"The resources on this planet will last 15.6B person-years which means if we send 5 million people there over time, we will have to prepare for their evacuation in ??? years"?

Obviously totally moot if Earth's resources aren't going to last that long, but just had that thought bubble up.

> The only driver that I can really comprehend is the desire for freedom and autonomy in less populated spaces

Exploration and seeing what's beyond seems to be innate in some people, not so much in others. Personally, if someone gave me to the opportunity today to "Sit in this rocket and get launched out into infinity and report back what's out there", I'd probably do it, and I'm sure I wouldn't be the only one. Curiosity would be enough for me to go.

Mankind will either spread further or die, this is binary. How much spread we can achieve or how much is even possible (ie due to limit of speed of light) is another topic, but if we want even with c being the absolute limit we can colonize milky way in maybe 100 million years if we want... in theory.