I'm late to this thread, but what I saw in the comment about dogs is how they foster social interaction and thus, build community - and they do so among more people, and in a different way, than children ever can.

Obviously there are people who isolate their kids and themselves, and people who isolate their pets and themselves. But walking a dog, or going to a dog park, puts you in public with people of all ages, family structures and backgrounds. You see, and likely meet and talk to, your neighbors, whether or not they have dogs (or kids) of their own. Taking your kids to play group or participating in their school activities only lets you interact with parents of children of similar ages - who may not be your neighbors at all.