We really do not owe people explanations.  As adults, we are free to make decisions and choices without explaining ourselves. If we are offered something, we can simply say “no thank you”.  If we decline an invitation, we can reply “no thank you”.  Often when we say “no thank you” or simply “no” people look at us as if they are waiting for an explanation–a reason that’s sufficient.  If we are invited to a gathering we don’t want to attend, then a  pleasant “no thank you” is all that’s needed.  We do not have to have a “good explanation” to say “no”.  We do not have to justify keeping our time to ourselves or our reasons to ourselves.  Sometimes we just don’t want to do it and that, my friend, is reason enough.  There is too much effort among us to control the use of other people’s time or to enforce our will on those we care about.  Sometimes, for what ever reason, we are not inclined to accept an offer or invitation.  Avoid the urge (and the pressure) to explain your self.  If pushed simply stick with your original answer (repeat if needed) without adding to it additional information or explanations.  No one has a “right” to know your reasons for declining.  You have a right to say “no” and leave it at that.  If nothing else, you can say “I don’t want to”.