You are 3. But you are only 3. You are testing and trying and fighting and questioning. You are Batman and Buzz Lightyear and a friend of the ducks. You are ours and you are perfect and you are maddening, all at the same time.
At its root, a tantrum is a result of frustration. Whether it is something the child can't have, can't do, can't express, or just can't tolerate, he is frustrated. His aspirations outweigh his ability. He wants to make all the decisions. He wants what he wants right now!