Why does our mouth "water"?
Basic biology: saliva in digestion is used to digest starches and fats.  It also lubricates food so that it is easier to move down the esophagus.  It is the first response your body has to food entering your digestive system and occurs in advance of food arrival. Pavlov's dog is a demonstration of this.

Rapid salivation occurs most often in two cases.  The easy example is seeing or smelling something like cake or pie, though this applies to any highly desirable food to the individual.  The case we think less of is that of starvation or even deep hunger.  In this case, even "everyday foods" cause rapid salivation as the body needs any calories it can get.