Well there is no theory behind this statement . It's only an assumption. Well here u have with own view:
When we think of good food we want to eat, we are
tickling our appetite. When we do so, our salivary
glands produce more saliva (hence our mouth
"watering" in lieu of the effects to our appetite.
Saliva contains enzymes and lubricants that help
you begin the process of digestion as soon as food
gets in your mouth. When you think of savory food,
your body takes this as a cue to start gearing up for
another round of digestion. Your 3 salivary glands
(parotids, sublinguals, and submentals) begin
excreting what they have stored and producing
more for the upcoming morsel. This process of
preparing to begin digesting food manifests as
"watering" in your mouth.
People with Sjogren's Syndrome lose their salivary
glands, therefore their mouths don't undergo the
same "watering" reaction that most people
experience in response to savory food.

Brain at work things!.why dont we salivate when we see the food we dont like?

.