Most of the more advanced (complicated? I don't want to be too species-ist) life forms on Earth reproduce sexually. I wonder how much of that produces faster evolution and is therefore selected for, and how much is chance? Like, say asexual reproduction dominates on some planet such that the apex creatures reproduce asexually, and it's only a minority of mostly uninteresting creatures that reproduce sexually. A lot of our behaviors would seem really strange; inexplicable, maybe. So much of our social interactions are ultimately originated in sex - and not just humans, but all sexually reproducing creatures. Birds. Cats. Other apes and our monkey cousins, of course. We get it; we recognize it, can sympathize with it. But if you're intelligent and your entire civilization reproduces asexually, and most of the other higher life forms on your planet do too, so that you don't have a huge biome of sexually reproducing, complex animals to study. I think it would be really weird to encounter humans.