There are three reasons for humans to seek new horizons:
philosophical; e.g. curiosity.
economical; e.g. to look for a new source of materials.
survival; because there is a threat in the existing environment.
The first, curiosity, is not enough anymore, mainly because the cost of this curiosity is seen as a burden instead of an investment.
The second, economical, is directed to short term profits. Investing in research that might return a profit a century away is something very few organizations do.
The third, survival, is an individual drive. Mankind has no "race-survival" drive, so even if we "know" there's a inevitable Mass Extinction Event on our path, there's no drive to deal with that threat.
So why should we try to colonize another planet anyway?
- Many people think this curiosity costs too much, especially when the cost will include death and mayhem.
- There is no reason to think it's going to be economical to go out there to mine for materials in our lifetime or the next 5 generations.
- Sending a (very small) group of people to another planet where they would have the same (or even more) chance of being wiped out by a disaster is not a race-survival policy, that would be just silly.
As much as I would like to see heros and heroins out there, I don't think there are enough logical reasons for "us" to pay and for explorers to do it.