If someone does something “small” that crosses a boundary of yours and you allow it… that opens the door for more boundary crossing in the future. You’ll know they have crossed a boundary because it will feel like a slight towards you.
The result? You eventually snap and you become a doormat or a total jerk.
If you are continuously defining the boundary and teaching people how you want to be treated, then resentments don’t build up. You will see that most people learn quite quickly in a new relationship. We feel shame when we aren’t socially accepted by others so we remember when we are corrected in our behaviour.
When someone crosses the boundary line in sport, they get penalized. They may complain, yell, or throw their arms up in the air, but they still go to the penalty box because they crossed the line.
Be the referee of your own life.
Side note: as long as you are very clear on where the line is (clarity is most important) you can be kind and gentle when setting a boundary. Always start with a kind and gentle approach, if it doesn’t work you’re guaranteed to get another chance to be bigger and stronger later.