While a good question, Eliezer_Yudkowsky has already thoroughly answered it in The True Prisoner's Dilemma.

His point there is, the values in the matrix are supposed to represent the participants' utility, rather than jail time, which accounts for your compassion for your friend. If it were simply prison sentences, your reasoning would apply, which is why EY says the true Prisoner's Dilemma requires convoluted, unusual scenarios, and why normal presentations of the PD don't make the situation clear.