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‘no-one will ever do or say anything to you that you don't invite them to do.’

: The above has to be the dumbest line I’ve ever heard. I truly hope I don’t need to explain why.

Not sure about that. There's some truth to it, although 'allow' would be a better word than 'invite'. Not my line, I heard it on the internet ;)
I guess the point is that how a person shows up effects how they are treated, hence clear and enforced boundaries actually help both sides...

I knew it wasn’t your line :-)