Hearing the idea of being adopted by Kate did sound rather nice in Sagan's head. For starters, it would mean that he would never need to return to his original family. Another bonus was the fact that it'd make it harder for him to be separated from Kate. He still wasn't very keen on visiting the doctor, but having heard Kate's idea made him a lot more comfortable with her other ideas, including being checked by a doctor. And even then, Sagan wanted to make sure he wasn't making Kate worried, and visiting the doctor would be a good start. He started showing visible attachment to her, one of the ways being his sudden craving to be held close to her, like a baby wanting to be held by their mother.