Is washing your hair daily to often?
Cleansing (in a good way) the hair and scalp is important, but cleaning the hair and scalp too aggressively or too often can cause irritation. Many cleansing products (shampoos/ rinses) cause the hair or scalp to become dry, irritated or flaking.
Shampoos/ Rinses that contain SLS/ SLES or cleaning your hair too often can lead to the excessive production of the sebaceous glands, actually making the hair and scalp oily/ greasier.
When the hair is washed less frequently (for example, from daily to three times per week), initially the scalp will create too much sebum (oil). As a result, the hair becomes greasier. After a few weeks, the sebaceous glands are accustomed to the new situation and will produce much less sebum. The hair will now be fuller and more shinier.
There is a trend of 'non-washing' your hair and scalp at all, but if you don't, grease and dirt will accumulate on your scalp and hair, which will make the hair "heavy" and it might not smell as nice. If hair is non-washed or combed for a while, ultimately dreadlocks will form.
Whether the hair is washed or not, it is important that you comb your hair every day (preferably every morning and evening).