Many pets are born with a predispositon to ear infections because either physical or pathological traits are passed through the breed lines.  Dogs are definetly more at risk for ear infections than cats.

Dog breeds with "floppy" ears (ears that do not stand straight up) such as beagals, basset hounds, labrador retrievers, golden retrievers, cocker spaniels, etc....are at a higher risk because ear flow to the ear canal to keep it dry (decrease the moisture from the wax) is severly impaired.

Dog breeds that grow large amounts of hair in the ear canals such as poodles, shih-tzus, maltese, terriers, etc... are at a higher risk because the hair acts as a foreign body that irritates the ear canal regularly.

Dog breeds that are known to have generalized allergies are also at a higher risk for ear infections. The allergies cause inflammation and irritation to the ear canal which begins the over production of wax. All breeds are susceptable to "generalized" allergies.