A personal limit becomes easier to respect when it stays clear under friendly pressure.

Limits are rarely tested by hostility. They are tested by good humour, by someone likeable asking a second time, by an atmosphere where holding firm would seem stiff and unfriendly. A limit that shifts under warmth is not really a limit, it is a preference that people learn they can talk you out of. Being predictable is kinder to everyone than a rule that depends on who is doing the asking.

Decide your answer before the conversation starts. Use the same words the second time, without adding a new justification, and let the repetition be friendly rather than reluctant.