“What can be the cause of frequent ear infections?”

There are many ways of getting recurring (chronic) ear infections. Causes range from allergies to sinusitis to ear injuries to bacterial infections from colds or flu. If you haven’t visited an Otolaryngologist (ENT) already, then I’d suggest you do so.

Depending on the type of infection you have whether it's external ear or middle ear could relate to your immune system which can be depended on your age as well. Other conditions may be anatomical that involve your ear anatomy and worth investigating. By testing for any difficulties with your immune system IgG IGA I GM and doing a temporal bone CT scan is where I would start.

Could be a lot of things. Could be your diet (i.e., too much sugar). Could be your anatomy. Do other people in your family have the same problem? Do you have an otolaryngologist that you see for these? If you're seeing general practitioner, that is something that can be quite often misdiagnosed. So, it may not be an actual infection. If you haven't already, I'd go see an otolaryngologist if you can. They can help get to the root cause if they're good and attentive. 

Middle ear disease, ear canal disease, mastoiditis