Also, incidentally, it isn't always about boundaries "setting someone off" -- although in instances of abuse it very well can be.
It's more about that you are individual people, allowed to have individual feelings about things, and you don't need to negotiate those feelings with a partner, even someone you are in a relationship with. We either accept someone as they are or we don't; likewise, we are either accepted as-is or we are not. If we are not, changing ourselves and our natures to suit someone else's vision of us is not healthy.