Neoteny is the retention of juvenile characteristics into adulthood. The constellation of traits that emerge when selecting for friendliness (foxes: https://wikipedia.org/wiki/Domesticated_red_fox) seem to be neotenous. So dogs are puppies, cats are kittens, and men and women are children.

Friendly people are selected for, by the benefits of cooperation, trade and civilization.

tl;dr the civilization of humans was their domestication