This is a struggle that most introvert have faced. Introverts tend to be quiet individuals. When you are too silent, you come across as unfriendly without meaning it. When you meet new people, you are quiet. When you are in group situations, you are quiet. When you are having a rough day, you are quiet. People mistake your silence for snobbishness or unfriendliness. They assume you are judging them, when really, you are terrified they are judging you. Understanding why they react to your quietness, helps dealing with those reactions.
There are plenty of clever responses to that question online, but most of them are either rude or make you look like a bigger freak. I prefer to say the truth. I find that educating people gets better responses and people accept you for who you are when you are comfortable and unapologetic with yourself.
There is a lot of positive to being an introvert
Quiet individuals are amazingly strong and sober when tough situations come in the way. Since they pass a lot of time on reflection, they know themselves really well and usually do not make rushed decision based on the immediate emotional response.
Introverts like being alone, this does not make them rigid and closed. On the contrary, the quiet individuals are observers and thinkers, tend to be very open-minded. They are always up for new ideas and activities.
If you remain true to yourself you will inspire confidence in others and create a network of strong, supportive people.