Many external factors may damage the hair, even if it is healthy to begin with. In fact, hair treatments (such as blow-drying, coloring or heat styling), the sun or wind, pollution, stress, or even smoking can cause damage to your hair.
These aggressions degrade first the hair's surface, then the cuticle, and finally attack the hair fiber. The hair loses its shine, and can become dry and brittle, or, conversely, may become oily.
It is therefore important to protect and maintain the hair, even if it is healthy.