Some here have mentioned these things:
1. Venting our problems to others who eventually use this against us.
2. Not using our manners.

These ARE behaviors that led to toxic relationships.

I had a relationship with my FOO where they encouraged me to vent my frustration with other family members, then they threw that in my face the first second when they turned on me. So, I got myself caught in a trap that led to toxic... see?

And, yes, if we all remembered our good manners, so much strife would be thwarted.