When your eyes water when you're nervous, it is typically due to an autonomic nervous system response. The autonomic nervous system is responsible for regulating various bodily functions, including those that occur without conscious control, such as heart rate, digestion, and tear production.
When you're nervous or experiencing strong emotions like anxiety, your body goes into a state of heightened arousal. This triggers the sympathetic branch of the autonomic nervous system, often referred to as the "fight or flight" response. One of the effects of this response is increased tear production.
The main reason behind the increased tear production is to help protect and lubricate your eyes during potentially stressful situations. Tears are composed of water, mucus, and oils, and they serve to keep the eyes moist and free from debris. When you're nervous, your eyes might become drier due to increased blinking or tension, and the body responds by producing more tears to ensure the eyes remain adequately lubricated.
Additionally, emotional tears are different from basal tears, which are produced to keep the eyes moist throughout the day. Emotional tears contain more protein-based hormones and are thought to help release stress and alleviate emotional tension, which could be another reason why your eyes water when you're nervous.
In summary, when you're nervous, your body's autonomic nervous system responds by increasing tear production to keep your eyes moist and protect them during potentially stressful situations. This is a natural physiological response and nothing to be concerned about.