I have always been told that washing your hair daily strips it of all it's natural oils, so your hair either goes into overdrive to replace said oils, thus leading to greasy, limp hair, or it stops producing enough oils and you wind up with crispy-fried hair. My old hairdresser said the average person (who doesn't put tons of goop in their hair daily) only needs to actually wash their hair once a week. I never quite make it that long, usually use some odd products at least twice a week which require some shampoo.