My mom's a dental hygenist and I've always wondered, seems that now there's an answer to this age old question:
"a certain ingredient in toothpaste called sodium laurel sulfate. It actually blocks sweet sensors. All the other taste bud cells in your mouth are firing away nicely, but the receptors which pick up the sweet sensors are not working anymore. Not only does it block the sweet sensors, it enhances the sour and bitter, so you get this massive influx of sour and bitter taste coming through the mouth."
Also, only some people smell that funky asparagus pee scent: