I don’t believe that I have ever seen this in person, but in toothpaste commercials, often, you see a loaded toothbrush going under the faucet.
Why?
Well, why wouldn’t you? Toothpaste (IMO, no cite) works better when wet. Now, it’s a bit silly to wet the toothbrush, then put toothpaste on, then wet it again…
I’ve always done it this way… bit more moisture helps the toothpaste to start foaming.
I do it. As other posters have said, it works better that way. Without added moisture, toothpaste is too pasty.
I find it weird that you don’t wet the toothpaste…
I do it because if you’ve wet the toothbrush before applying the toothpaste some of the water has gone.
But if you wet after applying the toothpaste you can shove a freshly wetted toothbrush in your mouth.
I am another wet-after-paster. Keeps more water in the brush.
I don’t do that, but I do rinse my mouse with water just before I use the brush. It’s wet enough and foams like crazy, so it must work just as well.
The mouse doesn’t object?
I thought mice cleaned themselves.
D’oh!
I have to admit to being boggled. The last time I checked the inside of my mouth, it was wet.
Yup, just checked right now: Wet.
So why exactly do I need to get toothpaste wet previous to sticking it in my mouth, again?
It’s all nice and fun being sarcastic, but I find that my teeth are not wet enough to get it going. And, it seems, so do all the people in the commercials and quite a few responses in this thread. Anyway, I’d rather get it started with plain tap water than work up a nice mouthful of saliva just to start brushing.
Wetter is better. 
Fresh tap water wet is way different than just a wet mouth when starting to brush the teeth. I run it under the tap before brushing. 
The ‘wet’ inside your mouth is dirty, smelly, and full of bacteria. I prefer to use the clean stuff from the tap to brush my teeth.
eta: Apparently human mouths are more bacteria-laden than any other mammal on earth.
Another person here who agrees; it just works better if you wet it. If you don’t wet it, you’re the strange one.
Do you wet your car before washing it?
Two wet things coming into contact are often better than one wet and one dry.
Now I want to use the term “paste-wetter” when I insult other people (although I’m a paste-wetter too!)
Who has moisture in their mouth when they get out of bed? My mouth is as dry as a desert salt lick. I’m brushing my teeth about two minutes after getting up.
The right way to do it is wet the brush, then put on the toothpaste, then brush your teeth.
What’s wrong with you people?