Why is toothpaste spicy?

The Greek philosophers called it "hedonism" -- the notion that things you like are good for you and things you don't like are bad for you. Toothpaste is right in the middle, a big zero, and most people never learn much of anything, so when they run across a toothpaste with a flavor they like, they buy more of that brand. The ones who take the trouble to learn about what they are doing don't use toothpaste at all because it has nothing to do with cleaning teeth. Brushing alone does that.