Problem is we live ridiculously short in intelligent species standards. Space colonization would only make sense if we break this cycle , we need at least practical immortality and then enough number of exoplanets to inhabitate billions. (as not everybody will die)
Before that, it's just leading the path. In that pov, very miserable we are, it's sad to know you'll not survive when it's a matter of time.
I can still see space colonization working without large lifespans, as there is the notion of robotic incubation, generation ships, etc.
I feel that the reasons that the Europeans colonized the Americas were varied. Firstly, it was discovered by sheer chance because of people hoping that they'd get to India by going West. After that, it turned out there was economic value to coming to the Americas. The we can do this because we can argument right now applies to bored billionaires, because of how much more vastly expensive space travel is compared to ships. For us to reach the point of why not, space travel has to be a lot cheaper, and granted, we may yet reach it.
I can see an Interstellar kind of scenario where people are just forced to go out. I can definitely see the scenario where we find out there's a sentient species out there, and we decide to get drop by for a visit (or they visit us and we're like we really should get off this one rock). I also feel like that surviving a potentially hostile sentient race would require being on more than one planet.