Frizzing of the Hair
A: Frizzing of the hair is generally the result of the cuticle layer of the hair being elevated, or roughed up.
When the cuticle is raised, the hair is more porous which also means that it can lose moisture and look and feel drier. When the hair's cuticle is laying flat against the hair shaft, the hair looks shiny and smooth.
Other people, conversely, have what is known as resistant hair, and the cuticle of their hair is especially tight. The hair stays shiny and smooth-looking, but is often hard to color and/or perm since it repels water.
The question of why some people have hair that tends to frizz and others don't seem to have a problem with it is simply a matter of genetics. The porosity of the hair (which is a function of the way the hair's cuticle layer lies) is one of the variables that make up an individual's hair type.
Normal porosity is generally assumed to be where the cuticle lays flat, but not too tightly against the hair shaft, and the hair will absorb water sprayed onto it after a few seconds of exposure. When the hair is more or less porous, you get resistant hair (that is water repellant) and porous hair (that is quick to absorb and release water - and is also more apt to be frizzy).
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See also:
How to tame frizzy hair
Dry and frizzy hair after blow-drying
Dry and frizzy hair after a sauna
The effects of humid weather on your hair