The right to refuse is incomplete if every refusal requires an acceptable explanation.

A refusal that must be justified is not really a refusal. If every no has to pass someone else’s test of adequacy, then the other person holds the actual decision and you are merely applying for permission to decline. Requiring reasons converts a right into a negotiation.

Practise the plain version: no, that does not work for me, without a supporting case. It feels rude the first few times and it usually is not. And when someone declines something you asked for, resist the pull to interrogate. Accepting a no without a receipt is one of the clearest signals that you respect the person giving it.