Brad Krause, the OCer improperly cited and exonerated for disorderly conduct while planting a tree on his own property, had the best answer:
Rights don't need a justification.
This really is the best answer for any but the genuinely "just curious."
All others need to understand that no justification is necessary. Its out-of-bounds to require or demand a justification. Police, anti-gunners, anybody.
Of course, any other answer you give to someone who isn't just genuinely curious, any other answer, will be negated, invalidated, remarked against by your questioner.