What makes sims special to me is the amount of time I have spent with them, and the accomplishments they have achieved. I think I need time to bond with them.
I regularly start a new family or save because I have a new story in my head. most of these are easy to leave behind and start over, but once I reach a certain point, I get attached to them and don't want to loose them. Usually at this point I have reached something like a family tree with a few generations, or completed a few aspirations, or earned a university degree.