The answer is overactive sebaceous glands. When these oil-producing glands attached to the follicle work overtime, the scalp and hair end up oily.
It's the opposite problem from a dry, flaky scalp, which points toward underactive oil glands instead. A fungal issue like Malassezia is more associated with dandruff than straight-up oiliness, and while diet and poor hygiene can make oiliness worse, the underlying cause is glandular, not circulatory.
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