WHY? - Unfortunately frizzy hair can be mistaken for unhealthy hair, some people have naturally frizzy hair and no matter how much they treat it or keep it trimmed it will still frizz. Frizzy hair can make some hairstyles look out of shape and unkept. Also frizzy hair doesn't shine much so can look dull and dry making it look like older hair.
SOLUTIONS - Rock your frizz, work with it, find a style that works with it.
You can use ionic heated styling tools like tongs or straightening irons that will smooth the hair cuticle layer to reduce frizz.
Try a salon keratin treatment which coats the hair with protein which will reduce frizz and makes hair soft.