A person starts sweating when a hormone gland in his or her brain senses the body heating up. This message is sent to sweat glands and they release sweat. When a person gets nervous or anxious, another gland releases epinephrine, activating sweat glands on hands and armpits.
People who live in hot temperatures or exercise a lot usually sweat less than people who live in the cold or don't exercise regularly. This is because the body has become used to its surroundings or routines.