Why is cheese so valuable to Ben Gunn?

Ben Gunn hasn't had cheese or many other things in the three years he's been marooned.... he dreams of it.

"Marooned three years agone," he continued, "and lived on goats since then, and berries and oysters. Wherever a man is, says I, a man can do for himself. But, mate, my heart is sore for Christian diet. You mightn't happen to have a piece of cheese about you, now? No? Well, many's the long night I've dreamed of cheese—toasted, mostly—and woke up again, and here I were."

becuase he hasnt eaten in 3 years he dreams of it. its like candy for a child

Benn Gunn was stranded on an island for 3 years, so he loves any type of food, not just cheese.