Why is it that when I think about sour candies, like Jolly Ranchers, I start to drool?
I think it is because you love them. The candies which are in different varieties are very sweet and they are readily available. Many people drool, just by thinking about them.
Sour flavors induce salivation, and thinking about them is enough to do it. Here are some boring science details: http://jdr.sagepub.com/content/69/3/838.abstract