When come back bring pie.
When the Euros landed in the New World, it was full of things (and people) they could use. That would not be the case if the sentient beings of one planet encounter those of another. Consider: If we humans were to encounter sentient beings, more primitive than ourselves, on a planet orbiting another star, we would have no use for them or their planet. We could not eat or drink or use as drugs anything that grows on that other planet, the biochemistry would be too different. Neither species could breathe the other’s air, so enslavement would require complicated life-support systems and would be more trouble and expense than it’s worth. Humans could not use that other planet for living-space without complicated and expensive life-support systems – orbital or free-floating artificial habitats, however expensive, are cheaper than terraforming another planet. And what else is there, that would make interstellar conquest worthwhile?
I recall Cowboys and Aliens – at one point one of the cowboys asks, “What do they want?!” And the Alien Hot Chick answers, “Gold!” “Gold?!” “It’s just as valuable to them as it is to you!” No, it wouldn’t be, however much their culture values gold, because any civilization capable of interstellar travel could get gold much more easily and cheaply by mining asteroids.
Resistance is futile; you will be assimilated.