Boundaries are not only about keeping people out as is commonly believed, they are also about maintaining love and care for yourself and the other person. Prior to this your relationships may have been about prioritising the others' needs above yours. This may have been the only way you knew how to be in the relationship, boundaries allow for another way where your needs are also considered and whether both parties are equally important. This protects from resentment, burnout etc and can actually save the relationship in the long run.