The Dictionary of Body Language

PUPIL DILATION—When we are comfortable or like something or someone we encounter, our pupils dilate. We have no control over this. When couples are at ease around each other their pupils dilate as their eyes try to soak up as much light as possible. This is why dimly lit restaurants are a good place to meet, as it naturally softens the eyes and makes the pupils larger—an effect that makes us relax even more around others.