When a dog gets wet after a swim or walk in the rain or is active in humid weather, those drooping ears can become breeding grounds for bacteria and yeast, which can in turn cause ear irritation in the form of red, itchy and even swollen ears.
If not kept clean and dry, a dog's ears can quickly become infected with otitis externa (a swelling of the external ear canal) or otitis interna (a swelling of the inner ear), which can only be resolved with antibiotics prescribed by a veterinarian.
Luckily, regular grooming and maintenance of your dog's ears are relatively simple, and being diligent in keeping them clean and dry can help you to prevent this kind of vet's visit. The simplest way to avoid infection is to thoroughly gently dry the inside and outside of your dog's ears with a cloth or towel when they become wet, but regular cleaning is also helpful, especially when you first notice symptoms such as redness.
Dogs who have had their ears gently touched and held from the time they were puppies, or dogs who have learned that ear cleaning is associated with positive reinforcement such as treats or praise, will be more open to this kind of grooming. If your dog can't keep still during an ear cleaning session, consider recruiting a partner to help hold his head steady.