Why did a dog’s conditioned salivation gradually disappear when food was no longer presented?

In the experiments conducted by Ivan Pavlov, the dog’s conditioned salivation gradually disappeared because the conditioned stimulus—such as the ticking of a metronome—was repeatedly presented without the unconditioned stimulus, which was the food.1

This process is known as extinction. Because the conditioned response is dependent on the association between the signal and the food, the absence of the food reinforcement causes the strength of the conditioned response to decline over time.1