We have about 100,000 hairs on our heads. Each hair shaft has three layers, with the cuticle, or outside layer, protecting the two inner layers.
Shiny hair is a sign of health because the layers of the cuticle lie flat and reflect light. When the scales of the cuticle lie flat they overlap tightly, the inner layers are protected from heat, sun, chlorine, and all the other hazards that can come from living in our environment.
When hair is damaged, though, the scales may separate and hair can become dry. Because the scales on dry hair don't protect the inner two layers as well, hair can break and look dull.