Why did Esau despise his birthright?

Genesis 25:29-34 tells the story about Esau selling his birthright because he is hungry and despised it. > And Jacob sod pottage: and Esau came from the field, and he was faint: And Esau said to Jacob, Feed me, I pray thee, with that same red pottage; for I am faint: therefore was his name called Edom. And Jacob said, Sell me this day thy birthright. And Esau said, Behold, I am at the point to die: and what profit shall this birthright do to me? And Jacob said, Swear to me this day; and he sware unto him: and he sold his birthright unto Jacob. Then Jacob gave Esau bread and pottage of lentiles; and he did eat and drink, and rose up, and went his way: thus Esau despised his birthright. Why did he (Esau) despise his birthright? Is there a good Christian commentary that explains what "despised" means, and why Esau would have despised it?