Why is it that when we encounter someone who appears to be anxious, we generally have a negative reaction, or ourselves become uncomfortable?

Viewing anxiety in terms of the fear of "being thrown back on oneself," this reaction seems to be a mystery. In living with chronic illness, I often feel my anxiety as the storehouse of energy that I have, but am unable to translate into use for various reasons. So, if anxiety was a sign of stores of unactualized possibilties for living, why would it have a negative connotation? What survival value does this impart?