I brought a new puppy into a household with a 13 year old dog. The senior dog, Pumpkin, was a toy poodle and the puppy, Ashley, was a mini. I did find that the senior dog was not at all interested in playing with the puppy. They got along OK but there was no bonding as Pumpkin was just too old for any kind of play. In your case, a dog in pain might not feel well enough to interact. This is not a criticism but, rather, so that you don't feel that something is wrong if they don't strike up a friendship. My senior dog did live another 4 years after we adopted the puppy. He was never in pain but was deaf, blind from dense cataracts and was developing a serious incontinence problem. It was our choice to eventually help him to the bridge at the age of 17. Had he been in pain, with no remedy for the pain, he would have had to leave us earlier. Each case is different, but I wish you well. Out of the two pictures posted I fell quickly for Millie. Please give Nicki some extra hugs from all of us.