The sensation known as "butterflies in your stomach" is a physical reaction to stress. It is part of the body's "fight or flight" response, along with an increase in heart rate, blood pressure and breathing rate, release of hormones and tensing of muscles.
The "fight or flight" response is an evolutionary response that comes from the days when humans were living amongst the food chain. When the brain senses danger, a series of events are triggered within the body. The "butterflies in the stomach" feeling is likely caused by the brain-gut axis, according to Greatest.com. There is a link of 100 million neurons between the gut and the brain. When the brain feels anxiety, it communicates that sensation to the gut, which can result in a slight fluttery feeling or all-out nausea. This connection being purely biological, the gut does not know the difference between being afraid or experiencing a first kiss.