Washing the inside of your dog's ear is sometimes an unpleasant but necessary task to keep your dog's ears healthy and free of yeast and infection. When you clean your dog's ears on a regular basis, you can remove built-up wax, debris, dirt, and keep the ear canal a healthy place instead of a place where yeast will likely grow and cause your dog pain and discomfort. Make cleaning your dog's ears part of your regular grooming, so he knows what to expect, how it feels, and he is always in great condition.
Having liquid poured into the ears isn't the most comfortable thing, so your dog may not enjoy this, especially at first. If he currently has an earache or ear infection, he's going to be uncomfortable and potentially in pain. Go slow the first few times to give him some time to adjust. Then try to do the task as quickly as you can to get it over with.