Relationships require sacrifice. Boundaries are placing limitations on what you are willing to sacrifice, how often, and for who.
When you are in relationship with someone (familial, friendship, parental, romantic, whatever) it's never just about you. You consider the needs, wants, and feelings of the other person because you care about them! And depending on who it is, their wellbeing is directly connected to yours! When the people I love most aren't doing okay, neither am I. I'm worried about them, trying to problem solve with them, maybe staying up late alongside them, or just losing sleep over whatever is happening to them.
I've been seeing and reading way too many things that encourage people to adopt an "all about me" attitude. If it means being uncomfortable or putting someone else's needs first, or saying no to something for someone else's well-being, it's presented as something to be immediately shut down. Ask yourself where would you be if people had been unwilling to go out of their way for you?
When people are new to things (boundaries) there's a tendency to over-compensate. As people learn boundaries, there's a knee jerk reaction to say no to everything and feel threatened by anything resembling sacrifice on our part. As India Arie said, "come back to the middle". Recognize that "no" is a complete sentence and also learn when it should be your response. It's up to you to choose. Just don't blind yourself to the reality that your wellbeing and your survival is inter-connected to the people around you.