When you explain your decision for a whole paragraph, it starts to sound like an apology. Two sentences is plenty.

Length signals doubt. A decision delivered in one or two sentences sounds settled, while the same decision wrapped in reasons invites the other person to argue with the reasons, since you have handed them a list of things to dispute. The explaining usually comes from wanting to be liked rather than from being unclear.

Say the decision, give at most one reason, and stop. If someone pushes, repeat the same sentence rather than producing a new argument. No is a complete answer, and saying it briefly is kinder than a paragraph that sounds like a request for permission.