This is a result of classical conditioning in humans. Classical conditioning pairs two stimuli in eliciting a response; one stimulus is a normal stimulus (unconditioned stimulus) and another is a learned stimulus (conditioned stimulus).
In this case, humans normally salivate in response to eating; perhaps pizza from Pizza Hut from example. Food aromas and restaurant symbols do not normally elicit the response alone; but since the restaurant symbol and food aroma is paired with eating a pizza (which normally elicits the response), then the human will elicit the response in just seeing the logo of Pizza Hut and the smell of pizzas, without actually eating.
