When one looks into the eyes of  someone they love, the world appears to brighten up a little. This phenomenon, however, is not merely a warm psychological feeling –– it is also rooted in physiology. 

Pupil dilation, or mydriasis, is a physiological response that the autonomic nervous system mediates. The sympathetic division of this system consists of a network of nerves that help regulate the flight-or-fight response. A variety of stimuli, such as danger, stress, and strength of emotion — including that of love — can activate the sympathetic response and cause the pupils to dilate, among other physiological changes. 

To understand why pupils dilate in response to such agents, consider the body’s physiological response to danger, like when a human needs to escape a dangerous animal in a forest. First, the visual cortex located in the occipital lobe of the brain assembles the visual image — a grizzly bear — processes the threat, and sends signals to the parts of the brain that activate the sympathetic nervous system. This activation causes the iris dilator muscle to contract, pulling the iris outwards and increasing the pupil’s size. With an enlarged diameter, the eye is able to capture more light and optimize the amount of visual information the brain receives. The person is then better equipped for survival. 

In a similar manner, the pupils dilate in response to increased cognitive activity, mental effort, and emotional arousal. Researchers have measured fluctuations in pupil size and reactivity in response to a wide array of stimuli. Although these changes occur on a minuscule scale, they have a profound impact on modulating our emotional experience. For attraction, this connection is thought to be due to the release of hormones and neurotransmitters in the brain associated with feelings of excitement and attraction. In particular, dopamine and norepinephrine are neurochemicals associated with feelings of excitement, attraction, arousal, and interest. 

Thus, whenever someone looks into the eyes of a person they love, there is a world of intricate psychological and physiological responses that are taking place. In more senses than one, love indeed appears to make the world a brighter place.