Our brains are primitive. Even though we live in this advanced world which is quite safe as opposed where our ancestors lived, our brains have not evolved as fast to understanding this. So our brains are designed to watch out for danger and keep us alive at all costs.
Now there's a portion of the brain, Amygdala, which has been tasked with this responsibility of keeping us safe from all danger. This is the part of the brain that takes us to fight /flight mode.
Now There is another part of the brain, which is the pre-frontal cortex, also known as the frontal lobe, which helps us be logical, be reasonable, take decisions.
Now usually when our brain senses danger, for e.g when you are crossing the street, and a car comes by. Our amydella is turned on! The first thing that the amydela does, is switches off the frontal lobe. Because if it doesn't, what will happen? The frontal lobe will start reasoning, and applying logic, like what speed is the car coming at, Should I jump right? should I jump left ? By the time you are deciding, the car can run over you. Hence the brains priority is to keep you alive at all costs, so the amygdala has more power and all it does is:
STEP 1: Switch off frontal lobe, i.e. any capacity to think or reason.
STEP 2. Fight or flight.
Now when your child is crying, your brain (being in the primitive form, that it is) can't tell the difference between your child throwing a tantrum or a tiger chasing you. It is the same thing for your brain, and it just says danger !!! Your amygdella goes on, frontal lobe off and you go into fight mode. You are literally trying to save your life. So you end up yelling, shouting and sometimes force handling or hitting your child.
The good news is that there are ways to break this pattern, and teach your brain how not to go into fight/flight mode everytime your child is throwing a tantrum.