Why does hair lightener damage hair?

Hair is made up of "layers". These layers contain the underlying pigments in hair. Pale yellow all the way to blue. Depending on how dark the hair is. The lightener (peroxide) eats through the blue, the red, the orange and so on depending on how light you're going. (This is also why some people get 'orange' hair when going lighter.) It weakens/destroys theses layers. Everyone has different hair textures and strengths. The ''levels" are different from person to person. And there are different levels of peroxides used for lightening. If making the choice to go lighter, go to a professional.