When you first experience your toddler melting down because you gave them the green bowl instead of the blue one, or you put the ketchup in the wrong spot on the plate, or you zipped up their jacket when they wanted to do it themselves, it seems completely ridiculous. I know it did for me.
But once I understood why it happens, it became a lot easier to stay calm and even laugh it off.
Their prefrontal cortex is still years away from being developed. This means emotional regulation and impulse control is not there. When something does not match the expectation they had in their head, they literally do not have the circuitry to emotionally adjust. The emotion floods in and they have almost no ability to regulate it.
They are not being manipulative or dramatic. They literally cannot do what you are asking them to do when you say "calm down"
This is why staying calm as the parent matters so much. You are their external regulator until their internal one comes online. When you stay calm, your nervous system actually helps regulate theirs.