Personally, I write as a hobby and am fascinated by characterisation. The Sims allows me to dream up characters and then throw them into a world and see what happens. I think attachment is inevitable, the same way we get attached to characters in a book/film/TV show.
I have repeated Sims occasionally, and I often create other favourite characters in Sim form. My standard Sim is Kit Yi who is currently in her fourth (at least) incarnation in my Sims 3 legacy. I have no doubt that I will someday recreate her for Sims 4.