Consider giving an explanation when:
-You believe the other will understand and NOT try to dissuade you from your decision or use it to manipulate you.
-It will add credibility to your response. For example, you're asked to attend an event for which you'll be out of town. Saying, "I'm sorry, I'm out of town that day" gives credibility to your 'no'.
Consider NOT giving an explanation when:
-You determine that it's not the other's business!
-The other is likely to use the explanation to tell you why you should/shouldn't do what they want you to do. For example, you explain that you can't help out because you need to be at another event. The other says, "Oh, you can stay for a little while and then..."
-Your decision will likely be used as ammunition against you at another time.
-The other may use this explanation to attempt to make you feel guilty.
How do I say "No" without giving an explanation??
Here are some options:
-"I'd love to help but I have other plans". No one needs to know what your other plans are. You can spend time alone, resting, picking lint out of your navel! It's no one's business what your other plans are.
-My calendar won't allow me to attend.
If you're uncertain and need time to gather your thoughts before feeling pressed to respond:
-I need to check my calendar and get back to you.
-I need to check with my spouse, babysitter, etc.
-Remind yourself that YOU are the one who makes decisions for yourself and that you have a right to decide what your priorities are without others