We don’t know what made this dog shove his nose into his friend’s mouth during a yawn, but we can assume it was the endless dog curiosity. Dogs are curious creatures, which is what makes them stop every two feet during a walk and smell anything they can put their nose into. Dog owners in the crowd are probably familiar with this sweet, albeit annoying, trait.
This is one option, but there’s another option we can think of: perhaps dogs, just like us humans, like to play pranks on one another. As children, we liked to interrupt our friends’ yawns by putting a finger in their mouth whenever we had the chance (didn’t you ever do that?), and it seems like this dog is doing something similar. The startled look on the blue-eyes dog is priceless.