I think sometimes people have trouble setting boundaries because they are afraid that it will hurt or offend the people they love. But that has rarely been the case in my experience.
Other people enforcing their boundaries is what makes them feel safe to me. If I know that someone will say “no” to me when I ask them to do something they don’t want to or can’t do— I feel much safer asking it. Because I never want people to over extend themselves for me. And it also means that I can trust when they say “yes” they mean it. I don’t have to guess if they are saying what they mean–I know. And it makes me feel secure.