While the comments about drying are true, there may be more reasons. Washing your hair is TERRIBLE for your scalp. This might not be a huge deal for some, but people who have a natural tendency to allergies and skin irritations suffer from shampoo. There's also that whole topic of washing your hair often actually makes your scalp produce more oil leading to dirtier hair and a vicious cycle of washing more and more often, but it's an entirely different discussion.