This is a very astute question and I'm glad to hear it from a 16 year old.
When it comes to friends and coworkers who would ask me why I carry a knife, I like to flip it on them using social pressure. When they ask me, I give them the most incredulous facial expression and say "You mean...you don't? How in blazes do you open anything? Your teeth? Please tell me don't use scissors on packages, because you will seriously injure yourself doing that." Of course I tune how heavy-handed I get depending who I am talking to.
You see, when they ask you why you carry, they are unconsciously subjecting you to social pressure and implying you are not like them, are are somehow deviant from "most people." You must remember that you are the reasonable one in the exchange, and frankly if you ask me, there is something wrong with them. So the key is to act like you're the normal one. Just don't be a dick about it, or your view will be dismissed as simple arrogance.
I do echo what everyone else has said about never bringing up self-defense. People like this often think that any capacity whatsoever to do violence is indicative of a deviant/violent personality, or that by being prepared for violence you must somehow be inviting it. Both views are of course stupid, but you can't change that, not in a single conversation anyway. It is especially advisable, as others have said, to never bring it up if the asker is a cop. Not only is it often get into tricky bits with intent and the law, but the cop's appraisal of you will usually sour. Cops are socially and psychologically conditioned by their jobs to resent civilians being able to defend themselves, because they unconsciously see it as robbing them of their purpose and identity (Protect and Serve). It also tends to make the more suspicious because a cop's daily life is filled with real criminals who are frequently armed and blurt out that it's for "protection," so they lump you and them together.