Well, I think some of it is temperment of the original dog, but also age.
My 2 year old GSD loved Fergus from the moment we brought him home. laid down to play with him, rolled him over on the floor with his nose and was so gentle given the 97 pound wieht difference!
Hamish, on the other hand, our grumpy old man teapot yorkie wanted absolutely nothing to do with him. And I can't say I blame him: the puppy jumped on him, chased him, stood on his back....you name it, he treated him like a littermate. Hamish was not amused. I will say, because of his advanced age (he was a rescue, we figure between 12-14) he probably cut the puppy more slack than he would of at 7 or 8 (when he probably would have nipped him for the transgressions)
But, in the end, you have to let them establish a pack order. In our house its clear: Hamish, Fergus, and poor Dante at the bottom (even though he could easily eat both of them for a light snack!). If you try to step in, disturb that order, you will bring yourself grief. Your pup may end at the top of the pack, you never know. Its about dominance and personality. My big guy wants nothing to do with leading the pack, he just wants belly rubs...while little Fergus is dead set on being top dog, protecting "mommy", and making sure everyone knows it.
Try and stay out of it as much as possible. They rarely will hurt each other, just nip and snap as if to say "back off, pal!"
