When your body is overheated, when you’re moving around, when you’re feeling emotional, or as a result of hormones, then nerves activate the sweat glands. When those nerves overreact, it causes hyperhidrosis. For instance, someone may only need to think of a situation that causes anxiety in order to break out in a profuse sweat.
Doctors aren't sure why people have primary hyperhidrosis, although it may be hereditary. Many people tell their doctors they’ve experienced excessive sweating since they were a child.