An ordinary limit should not require an extraordinary story to support it.

A plain no is complete on its own. There is a habit of building a justification large enough to be unarguable, which teaches everybody that limits need proof and makes the next one harder to state. If you can only decline when something dramatic is happening, most of your ordinary preferences will go unmentioned. Reasons are optional; the limit itself is the point.

This week, decline something in one short sentence with no explanation. Not this time, I am not free, I would rather not. Resist filling the silence afterwards, and notice that nothing further is usually required.