A boundary is easier to maintain when it does not require a fresh argument every time.

A limit that has to be defended on each occasion is really a negotiation with a name. If every request reopens the case, you end up spending your energy on persuasion rather than on the thing you were protecting, and eventually giving in becomes cheaper than winning again. Durable limits work because they are stated once, applied consistently, and not offered as a topic. Consistency does more of the work than any explanation.

Pick one limit you keep defending and decide on a single sentence for it. Use the same words each time, without new reasons. Repetition without argument tends to end the discussion faster than a better justification.