Simple dry skin is the most common cause of dandruff. If this is the cause of yours, you’ll likely have areas of dry skin all over your body. Another frequent cause is irritated, oily skin (seborrheic dermatitis), which is marked by red, greasy skin covered with flaky white or yellow scales.
Not shampooing enough can be a culprit. If you don’t regularly wash your hair, oils and skin cells from your scalp can build up, causing dandruff. Other causes can include psoriasis, eczema, contact dermatitis (from sensitivity to hair-care products) or a yeast-like fungus.
If you are experiencing dandruff flaking or an itchy scalp, see your doctor, who should be able to determine its cause by looking at your skin.
— Source: The Mayo Clinic