Because of the anatomy of the ear in some breeds of dogs. It's longer and deeper than in humans, for example. And in some breeds, covered by an ear flap that makes the inside of the ear canal warm and moist. Like an incubator.

Another reason: dogs have a lot of mast cells that can release histamine and other allergic chemicals. In other words, it's common for dogs with allergies to have itchy, irritated ears. It all comes together; the allergies irritate the ears and in response they produce more ear wax. Because of the extra ear wax and inflammed ear canals, and because the ear canals in many dogs are extra warm and moist because of anatomy, you can understand why so many dogs keep getting ear infections over and over again. More... It's pretty easy to tell when your dog has an ear infection. They shake their head. They whine. And when you give hugs, you can smell the special odor of a yeasty, bacterial, ear infection. And, of course, if you take a close look, you will see that the ear canal is hot, red, inflammed, and full of goo. If you touch the ear it will often hurt. Or elicit groans. I went to vet school to learn this great stuff.

In case you didn't see my smile, that was a little joke. But here's something serious. You can't just assume what I just told you is true ... for your particular pet. Each case is different. Or complicated by other factors.

Possible other causes include foreign bodies in the ear canal, masses in the ear canal, drug sensitivities, food allergies (which are different from other allergies), thyroid disease, immune disorders, parasites (there are several different types besides the common ear mite), metabolic disorders including diabetes and cushings disease. Other factors include "swimmers" ear, excessive hair growth in the ear canals, excessive hair removal from the ear canal, excessive bathing, sensitivity to harsh ear wax cleaners. Oh I forgot to mention yeast and fungus in addition to bacteria. And sometimes the bacteria is the type that doesn't cooperate and die even when you're using a powerful antibiotic. Hoo Boy.

I bring this all to your attention to underscore the importance of a good exam and probably some lab work to try to narrow down all the possible causes. Your vet will want to have a good look down the ear canal. It may be necessary to sedate your pet in order to clean out the ear well enough to get a good look. Or maybe he or she will have you come back after a few days of treatment to look again when hopefully the ear will be improved