Why is conditioning also called learning by association?

Consider the original experiments of Ivan Pavlov. He would ring a bell and then feed his dogs. The dogs always heard a bell just before they were fed. Therefore, the events of a bell ringing and being fed were associated in the minds of the dogs. Therefore they were conditioned to expect food when they heard a bell, and as Pavlov observed, their digestive enzymes would be secreted when they heard a bell, even if he did not feed them. The bell alone was enough to produce the conditioned response of preparing to digest food.