Why do I get dryness behind my ear?

Seborrhea. This can represent seborrheic dermatitis, psoriasis, or allergic contact dermatitis.Cthis is treatable with prescription cream or lotion. See dermatologsot forve aluation and treatment.
Not sure. Hard to say why you have it. However, others develop eczema behind their ears. To make the correct diagnosis and get the correct treatment, i recommend an appointment with an ear, nose & throat physician or dermatologist.