How is hope like a "dog's ear when it hears the refrigerator door open?"

First of all the phrase is a Simile, comparing hope to the action of a dog when it anticipates what will be coming out of that refridgerator once it opens. The phrase is essentially comparing how we (humans) dogs etc anticipate, yearn, hope for something in life. When a dog hears the refrigerator door opening, its ears perk up, mouth begins to salivate, and its eyes are wide with expectation. It is the same basic manner in which humans act in anticipating, yearning for something.