It's just because YOU CAN'T HANDLE THE TRUTH!
Son, we live in a world that has rocks. And those rocks have to be climbed by men with guns, and big bulging muscles. Who's gonna do it? You? You, Lt. CForrester? I have a greater responsibility than you can possibly fathom. You weep for wasted time at the start of a movie and you curse the Marines. You have that luxury. You have the luxury of not knowing what I know: that those cheesy commercials, while tragic, probably saved lives. And my existence, while grotesque and incomprehensible to you, saves lives...You don't want the truth. Because deep down, in places you don't talk about at parties, you want me on that rock. You NEED me on that rock.
We use words like honor, code, loyalty, popcorn...we use these words as the backbone to a life spent climbing something. You use 'em as a punchline. I have neither the time nor the inclination to explain myself to a man who rises and sleeps under the shadow of the very rock I climb, then questions the manner in which I climb it! I'd rather you just said thank you and went on your way. Otherwise, I suggest you pick a rock and start climbing. Either way, I don't give a damn what you think you're entitled to!
(half an old cookie to the first person to name the source, as that's pretty bloody obvious if you haven't been living in a cave all your life)