I get anxious about a LOT of things. Social situations, obviously, because they're so unpredictable, but also crowded areas (unpredictable), going to a new place (unpredictable)... I think you can sense the pattern here.
Most people find unpredictability to be somewhat stressful, but apparently people with ASDs find unpredictability even more stressful than most. Possibly this is partly because we can't predict many things that others can predict, such as what another person will think or do in a given situation, or even what we will think and do in a given situation. SO much information is coming at us, and we don't know how to process it, so we just overload. So the world is inherently more unpredictable for us (in social contexts, anyway - we can be quite good at predicting other patterns, I think), and therefore we become more anxious.
That's my theory, anyway.
And medication does help - I'm on antidepressants and an antipsychotic, and I find it much easier to function and take on new challenges now. The unpredictability of the world still bothers me somewhat, but I overload less now, and I can filter out some of the information (possibly not the information I should be filtering out, but it still helps).