I have been carrying for over ten years, and I have never had anyone ask me, "Why do you open carry?" The questions are usually as follows:
"What's the gun for?" And I reply, "For launching projectiles at sub/super-sonic speeds."
"What do you need a gun for?" And I reply, "I hope I don't need it -- but I'd rather have it and not need it, than need it and not have it."
"Why do you carry a gun?" And I reply, "Because it's more convenient and less cumbersome than carrying a box of grenades."
"What's with the gun, dude?" And I reply, "Bullets, dude."
Of course, I try to be cordial and friendly and, if the person will hear me, I like to explain the legality of law-abiding citizens carrying firearms for the protection of themselves, others, and the state.