Psychology and body language studies can teach us a few things about making a good portrait. Here's one of them. Whether you realize it or not, your pupils dilate when you are attracted to someone or excited. This is not the ONLY time they dilate, obviously, but it is one of those primal signals that says, "Hey, I like you" or "Who turned out the lights?". Knowing this, you may now notice that photographs where you seem most attracted to the subject might just be those where the pupils are most dilated. They use this trick in magazine photography quite a bit too, actually making women appear more attractive by digitally enlarging the pupil (and various other body parts).
Take a look at these two sets of eyes. Both were taken with the same lighting, of the same person, within seconds of each other. Which do you feel more "drawn" to? Probably the one with the larger pupils.The reason why the pupils are more dilated in the second image is that I turned the subject slightly away from the main light so that it was not so bright in her eyes - which naturally allowed her pupils to dilate. It wasn't that she was attracted to me, it's just that her eyes needed to adjust to the lower light level - but the effect is literally the same. So here's the tip: keep your main light source either out of your subject's eyes as much as possible, and/or lower your modeling light and surrounding ambient light level as much as possible - while still allowing for reliable focusing. When light levels are low and/or there is less glare in their eyes, your subjects pupils will dilate more - creating the subliminal message of "I like you", which certainly makes me enjoy a portrait more :-)