I got Brody when he was 14 months old. He attached very quickly and became mommy's baby. He was very willing to play with my older son's Dachsi mix, but 'captured' mommy's lap every time Lady wanted to sit in Nana's lap...which she LOVES to do! After puppy sitting for a friend's Shih Tzu, Brody was so very sad and forlorn, having had a playmate and companion with him daily for almost 2 weeks, that I went looking for a friend for him. Three months after bringing Brody home, we took Brody to meet (golden) Mia, the two hit it off and she came home to live with us. Both Brody and Mia were 17 months old when we got Mia...they are 6 days apart in age. Both were 3 in March. They get along great...love each other soooo much. Brody got up twice today just to walk over and kiss Mia. Brody doesn't lift a leg to pee, either, but Mia lifts her right leg just a little bitty bit when she pees.
I think a lot has to do with the pups themselves, and how you handle it.