1) Because I am a lawfully armed citizen and refuse to be frightened into voluntarily sacrificing my lawful right by a populace of ignorant sheep who wish for me to join the herd.
2) Because I am a proponent of the 2nd Amendment and realize the hiding my firearm under clothing only serves to lower the visibility of the issue. (I like to use their question as an example: "If I hadn't been openly carrying, would you still have asked me about my support of the 2nd Amendment?".)
3) Because if people don't exercise their rights, what's the point of having them in the first place?
These are just a few of many.
What I avoid like the plague is replying with smarmy, wise-ass remarks.
A person inquiring about OC is most likely not informed about the issue, curious, or possibly thinking about working up the courage to try it themselves in the future.
Why would anyone wishing to promote the 2nd Amendment teach people asking about it that folks who carry guns are all smart mouthed a**h*les?
As a very visible representative of the firearms community, I go out of my way to always be polite and treat every question as an opportunity to sway people to my side, not alienate them by being a jerk.