Having flexible external and internal boundaries are vital to healthy living. A strong yes and no shows you matter to you and others. It so important that you say NO and stop the things, people and situations that don’t serve your highest being and a strong yes to things that nourish your soul. It is not selfish to have boundaries with others, its a sign of self respect for yourself and others. That way others know where they stand with you, will treat you in the way you want to be treated, it will allow others to understand how they can expect to be treated by you. It can stop people will ill intentions from taking advantage and allow you to be open to all people for new friends, opportunities etc.
Boundaries are always changing and not set in stone. If you are unsure, take some time to reflect and listen to intuition on what feels right and set it. Except negative feelings from love ones and be open to chat that the boundaries are not to harm or dishonor them but to help the relationship grow. If someone really does not like and acts out anger towards them (then consider this a red flag and distance yourself or walk way)
Remember boundaries are important act of self love and necessary for healthy relationships with others and yourself.