Practice stating your decision and then being quiet. It is your decision to make. You can share a reason with someone if that’s what you want to do, and you don’t ‘owe’ anyone an explanation. Sometimes offering up a reason to someone puts them in the problem-solving mode of how to overcome that reason. If that’s what you want at the time, then by all means do it. If you have definitely made up your mind and don’t want to discuss it, then consider not opening that door.