Why do I sweat when I'm nervous?

Norepinephrine. There is an increase in the release of the chemical norepinephrine with anxiety and one of the results is an increase in sweating.
Sweatgland/adrenalin. The sweat glands are controlled by your autonomic vervous system in which high adrenaline state causes you to sweat. Emotionally induced sweating typically involves palms/soles/axilla areas. Your sweat glands maybe a bit sensitive to Adrenalin than usual, thus when anixous (adrenaline released/increased) you sweat more...If excessive, consult doc for treatment. Hope this helps. Good luck.