A passionate kiss acts like a drug, causing us to crave the other person thanks to a neurotransmitter called dopamine. This is the same substance involved in taking illegal substances such as cocaine, which is why the novelty of a new romance can feel so addictive. Dopamine is involved in sensations of reward, making us feel intense desire that can lead to feelings of euphoria, insomnia, and loss of appetite, and it is only one actor in the great chemical ballet happening in our bodies.
And then there are physical changes. A kiss can cause our blood vessels to dilate, our pulse to quicken and cheeks to flush. Our pupils grow wide, which is likely one reason that so many of us are apt to close our eyes. In other words, the body’s response mirrors many of the same symptoms frequently associated with falling in love."