We all know reasons for human expansion into space: preventing human extinction, garnering resources from beyond Earth (rather than tearing the Earth up), raw human curiosity, helium3, solar energy, enabling the instantaneous woldwide downloading of funny cat videos, .etc. I’ve though of another.
Judging by the nastiness of humans to their brethren, we need to be able to isolate breeding populations of humans from each other so that they may evolve into something better/nicer.
A population in a hazardous, potentially lethal environment is just what spurs natural selection. The individuals MUST work together or perish.
We’ve always been a self-competitive specie – fracturing into smaller tribes and migrating for food, or room, or just to get away from the assholes who run things. Now we’ve run out of room.
We go into space, or we choke in our own waste, like yeast in beer.