I got this from my friend, Renato! Brushing your teeth makes your entire mouth feel refreshed, and it also makes the feeling of singing more refreshing!
My guess is it's because brushing your teeth rids your body of "germs". Don't you feel so much better after being sick? Same thing! It's temporary of course because bacteria will continue to build up, but feeling better about yourself and your singing is way worth it!!! Even if it might only be for an hour!