I do think shyness is a common response in many with ASDs, but I would distinguish that from an essential trait.
I believe we learn to keep to ourselves once we start to realise that we are different, and it gets reinforced by so many others around us who fail to understand who we are, how we think, and how we engage with the world. We start to doubt ourselves, and become alien. Were we not subjected to these early messages, I'm not so sure we'd be all that shy. At least not with any more frequency than an NT.
I believe we learn to keep to ourselves once we start to realise that we are different, and it gets reinforced by so many others around us who fail to understand who we are, how we think, and how we engage with the world. We start to doubt ourselves, and become alien. Were we not subjected to these early messages, I'm not so sure we'd be all that shy. At least not with any more frequency than an NT.