Dropping the reason is the whole thing and it took me a year to believe it.
A reason converts your decision into a problem for the other person to solve. Say you are busy, and they will find you time. Say a deadline, and they will move it. None of that is malicious; you handed them a puzzle and people solve puzzles.
The test that convinced me: try one no with no reason and watch how rarely anyone asks for one. I expected to be challenged constantly. In about a year it has happened twice.