This is a big claim to make, and it’s mine, not the author’s, he’s just one of the many ways to instantiate it.
What is this biggest mistake then?
Not listening, and going against the ego of other people, so they don’t listen they want to speak just as much as you do and until you let them be heard they will be blocked.
You can’t try to force other people to change, you have to treat them like humans. quote
This may sting, and me saying it this way probably makes you resistant to the idea.
Oh well! Grow up.

I like what this person said, and how he compared couple’s therapy to politics and putting out the reason why couples wouldn’t work and comparing it to why any interaction between people wouldn’t work.
He says you need to create a space where people to feel heard, because you cannot change anyone’s mind, you can only give them the space to do so.