The Appearance of Thicker Hair....

Why does your hair look thicker after you blow dry or color it?

When you look at a strand of hair under a microscope you will see something that resembles a fish scale. If you have a virgin head of hair that has never been touched by a chemical process, this scale will look smooth, just like it lays neatly against the fish close to its side. Highly bleached, tinted and permanently waved hair will have all the scales up and open. The coloring mixture opens the cuticle and colors inside and out. This is why you appear to have more volume in your hair when you color. The texture appears thicker and fuller because the scales are wide open. (HairFinder)

..... And if our cuticle is open our hair is prone to all kinds of damage!