When you become aroused, your brain sends out the order to send blood to the penis (and, if you are a woman, the same happens down to the vagina). The spongy, cavernous tissue fills with blood, which creates an erection. Why exactly the blood lifts the penis (it seems counterintuitive; you think it'd weight it down!), I don't exactly know, but it does increase sensitivity, since the blood vessels and nerves are engorged.. . Hope that helps!