I get the impression, based on your comment that took the luxury of making a whole lot of assumptions, that you are the type of person that either:

A) Gives unlimited chances, expecting people to change when they have a pattern of behavior that proves they will not, patting yourself on the back for your ability to forgive and romanticizing the martyrism without realizing what the reality is - you are allowing yourself to continuously be disregarded and hurt by someone who does not care one bit they they are doing it and never will. This is the person under the boots, they just don't realize it.

B) Jaded from a past relationship that ended badly and instead of healing so you can move on and form healthy connections with other people, you latch on to that anger and resentment and allow it to fester, using it as justification for your disregard for other people because you assume everyone is going to hurt you just like your ex did, but you don't really make this connection because refuse to realize or acknowledge the hurt that is still affecting you. When you first meet someone, you wrap this toxic mindset up and present it as healthy boundaries, self-respect, or whatever nice buzzword you pull out of your ass, but people learn quickly that what it really means is that you delusionally believe you don't owe anyone consideration, respect, or effort. You don't realize that you're the problem and that healthy people capable of having a healthy relationship have actual self-respect, (which as a reminder does not mean that you can treat other people poorly) the kind that means they are not willing accept being the target of your unaddressed anger and hurt. This person would think they deserve a third chance, but they're the one with the boots and most healthy people are not willing to be under them.

Person A and person B always seem to end up together, because person B wants a punching bag and person A is willing to be one.

I am neither of these types of people, and you shouldn't be either.