Pupil dilation actually occurs for many reasons. If you've ever stood in front of a mirror and turned the light on and off, you may have noticed the effect it has on your pupils.
Your pupils dilate in the dark because this allows more light to enter.
Some drugs can also cause pupil dilation, but among the most interesting cause is pleasure, both physical and psychological.
This includes when you find someone attractive. The mind likes to look at attractive things. Our minds also see dilated pupils as being attractive. What's interesting is that if you find someone unattractive, your pupils will contract.
It can be a subconscious measure of how much you like a person. This isn't exclusive to people, though. The pupils will also dilate when looking at anything visually pleasing: Scenic nature, cute animals, good food, and even the right kind of technology for some fanboys.
There is a whole study devoted to pupil size as an indicator of emotion, known as pupillometrics. In a series of studies conducted by Eckhard Hess, a biopsychologist, and his assistant James Polt, a few things were concluded. They showed men photographs of women.
Some photographs had women with dilated pupils, and others didn't. The men generally favored the photos of the women with enlarged pupils.
A study at Edinburgh University also proved this also applies to women looking at men. In the end, it was concluded that the attraction to dilated pupils is factual, but subjective.
Though people with dilated pupils are generally regarded as attractive, the pupils of people viewing them will typically only dilate if the subject is of a corresponding interest based on sexual orientation.