As such, dogs form attachments to those humans and do not like it when those attachments are in any way altered or threatened.
See exhibit A:
a puppy cuddling up under the arm of its owner.
Now then, the older dog is used to cuddling up under the arm of their owner. But with the new puppy in place, this attachment is threatened.
Check out the problem solving that the older dog takes on:
That's right—this big ol' pup both brought over a soft dog bed for the younger puppy and then physically moved the little one from the couch to the dog bed so that it could grab its usual spot, draped over its human.
Have you EVER seen a more polite display of jealousy?
Do you think the older doggo knows this and is trying to gently claim its territory?
And the little guy's look at the camera at the end?
But the older pup is plenty happy.