clipped from: www.livescience.com   
No one knows why 2-year-olds have temper tantrums, but most of them do. It starts with mild anger over something simple but then quickly escalates into full blown fury dramatized by screaming, fist pounding, foot-stomping, and screaming.
Pediatrician Harvey Karp, author of "The Happiest Toddler on the Block," and an expert in getting babies and toddlers to quiet down, claims that tantrums are an expected product of human development. He sees our little darlings as less-evolved savages driven by instinct and emotion, not thoughtful reasoning, and he suggests it's our job as parents to civilize them into Homo sapiens.
His method is to speak in short phrases that reflect the primitive emotions of the child ("You are angry") rather than addressing the adult modern Homo sapiens situation of the moment ("Please stop. Big girls don't scream in stores.")