As it turns out, whenever you see someone you like, your pupils do dilate. It’s a physiological response that you have no control over and can easily betray your feelings to onlookers. And it’s one of the strongest physical responses the body gives off to convey their emotions to a person they are attracted to. A strong eye contact and your eyes’ blink rate can also very easily indicate how you truly feel.

The word "pupil" means "doll-like reflections of people" in Latin. So when you see someone you like, your eyes dilate as if they are trying to see as much of that person as possible.