I worked at a dog day care and there were 2 jack russels a boy and a girl that were brother and sister. The boy would do that same thing to his sister. After a while that other dogs started to do it. SHE LOVED IT. Sometimed she would fall over if the other dog did it for a long time and she was leaning in and they stopped. My dogs will do it every now and then what they are really happy with each other!!!