My guitar gently weeps
Join Date: Jan 2013
Location: Northeastern US
Everyone experiences anxiety (and all emotions) to varying extents. Not everyone "has" social anxiety, as in the diagnosis, but almost everyone experiences anxiety in social situations from time to time. Emotions are normal and healthy physiological responses to our environment.
Pple with SA tend to have some beliefs that reinforce and perpetuate that anxiety beyond "normal" manageable levels. It moves into diagnosis territory once it begins to affect functioning. Then apart from "just SA" pple, you have those with other stuff going on such as trauma, pd's and other comorbidities that further reinforce anxiety and avoidance.
So yes, everyone experiences anxiety in social situations from time to time for a variety of reasons. Those whose level of anxiety affects their functioning may need a little (or extensive) help bringing anxiety back into manageable levels.
As to why some have these issues and some don't? We all have life experiences that shape the way we view the world and pple. Some of the beliefs we pick up work for us, some of them really ****ing don't. Really the luck of the draw what and who you're exposed to and what beliefs you pick up (until you're older and become more consciously aware of the choices you have in the matter.) Had loving, validating parents and/or were accepted by your peers for the most part? Unlikely your anxiety with pple will get out of control. Had a ****ty upbringing and/or chronically experienced bullying, invalidation, rejection, etc.? Yup you're likely to have some struggles with social anxiety later on. Then there's an entire biological side to it that I personally don't put too much stock in, so I can't really speak to it.