Let us get the most obvious reason out of the way first: cleanliness. Dogs are not exactly known for their use of Q-tips, but they are geniuses at using their tongues as washcloths.
Your dog might be licking their buddy’s ear because they sense some wax, dirt, or a weird smell. It is a deeply ingrained grooming instinct. Think of it as a favor: “Hey, you missed a spot back here, pal.”
But here is the ironic twist. Too much of this “help” can actually cause ear infections. So your dog, in their infinite wisdom, might be spreading moisture right into a dark, warm cave—perfect for yeast parties. Nice going, Fido.