What's the difference, right?

A reaction is usually an emotionally-neutral response to something or someone. An over-reaction is NOT emotionally neutral.  Not. At. All.
And why do we over-react?  Because we've just been triggered....by words, a look, a certain tone or silence.  And 9 times out of 10, these triggers have an origin in our early years (typically childhood or adolescence).  One time my husband and I were sarcastically bantering with each other and at one point he said, "don't be such a dummy." In general, this is not a cool thing to say...but my prefrontal cortex knew he was joking because of how we were bantering. However, my limbic system (emotional brain) wasn't so sure and I immediately over-reacted with RAGE!  He was surprised by my over-reaction and honestly, so was I.  Trusting my prefrontal cortex, I knew my husband did not believe I was stupid or dumb.  But once I calmed myself, I asked myself what was THAT all about?  I closed my eyes, started some deep breathing and waited for something to emerge...and something did. A memory. As a child I was a bit embarrassed having a mother who was illiterate, especially when she forced me to read all the time. Even as a little girl, I thought her forcing me to read was hypocritical since she wasn't trying to learn how to read herself. And she didn't hesitate to use phrases like "don't be stupid" quite a bit. As an adult, I understand she was doing this for my own good to motivate me to not be like her. Once this memory emerged, I simply visualized my adult self telling my child self "you are not stupid....you are very smart...mom has a lot of her own shame that she carries with her and it has nothing to do with you." After realizing my rage toward my husband was related to this childhood memory and doing this brief exercise, I felt released from that trigger and was no longer angry at my husband.