I grew up as a shy/quiet child. It was my nature, but it was frowned upon to some extent by society, but particularly by my family. I felt rejected. I tried desperately to change, but to no avail; and this made me anxious.
We develop as a combination of nature and nurture. I think much of the tendency towards social anxiety is in our nature, but it is only activated by the nurture aspect – specifically when we aren't taught to accept and embrace these aspects of ourselves, but rather that we need to reject them and change. And there lies the mischief.
In my case I believe my parents thought they were doing me a favor in "toughening me up" so that I would be ready for the big, bad world. They had no idea about the quiet strength of acceptance. They thought I would be eaten alive by the sharks out there, but I just wanted to be the pilot fish that swam unconcerned among the sharks, perfectly capable of looking after myself. 
At least now, in adulthood, we can work on accepting everything about ourselves - all the things we have viewed as the Good, the Bad and the Ugly, removing those artificial labels. We can embrace being quiet (even shy) as a strength rather than a weakness.
Quietness and introversion come with many benefits. Not least is that we pay attention to the environment around us, and to the verbal and non-verbal cues from others. We learn to view things from many different perspectives and viewpoints, coming to a considered conclusion, rather than just bulldozing ahead thinking our view is the only one and the right one. We look before we leap. We are typically compassionate, caring, considerate and supportive to others.
We can deal with sharks and bullies in the world from a position of quiet strength and acceptance of ourselves. And to extend the Clint Eastwood reference a bit further... we can tell the anxiety bully: "Go ahead. Make my day".