I'd modify that to once you have kids, you might understand. It's for sure not a given and there are a lot of really terrible people out there who probably shouldn't have the kids they do.
I don't think I'm a terrible person. My kids are healthy, mostly happy, seem to be well adjusted. I support their physical needs, do my best to support their emotional needs. I read to them, changed their diapers, taught them how to ride a bicycle, help them with their homework, kiss them goodnight. But I still don't understand. I know most parents have this whatever it is, but I'm still wondering.