How do boundaries work?
Boundaries are your first line of defense. They are how you protect yourself from unhealthy relationships and situations. They are how you keep yourself and your stuff separate from other people and their stuff. Boundaries are NOT ways to control other people. That isn't what they are for and they don't work like that. You can't control other people. They will do what they want to do for their own reasons. They are simply the rules you've created for yourself regarding the way you will allow others to treat you.
Because of this, boundaries are a critical part of self-care. They require you to respect yourself enough to say, "No." They require you to recognize your own value, to stand up and say, "I don't deserve that. I don't like that. I don't want that." When boundaries are practiced regularly, self-worth and self-respect come naturally. These things are a natural by-product of behaving in a manner that acknowledges your own value and requiring others to do the same.
This is why boundaries are not negotiable: because your value is not negotiable. A person who does not respect your boundaries is not valuing you as much as you deserve. A person who tries to negotiate or manipulate around your boundaries does not recognize your worth. If you allow this, you're telling them that you don't, either. We teach others how to treat us, so it's very important to teach people that you mean what you say and that you respect yourself enough not to put up with being disrespected or treated badly.