Why do dogs put their ears back?

According to the APSCA, when a dog pulls his ears back slightly, he is interested in being friendly; however, ears fully flattened against a dog's head indicate he is feeling frightened. This body language is also a submissive gesture.

Dogs are expressive, and their ears display a great deal about their emotions. The ASPCA states that when a dog is relaxed and content, his ears are in their natural position. When a dog hears something of interest, his ears perk up and he moves them in the direction of the sound. Dogs who hold their ears up and forward are often indicating aggression.