The pupils of the eye dilate and constrict often throughout each and every day. Even for blind people! It's a very normal and healthy function of the eye which aims to make sure our eyesight is the best it can be. Most people notice that in dark rooms, the pupils dilate and this is to capture more ambient light so you can see what's around you. Very bright rooms will have an opposite effect. Medications, and also dietary intake can have effects on pupils. Children, for example, who become briefly very hyperactive after eating lots of sugar or drinking soft drinks and cordials often have dilated pupils as well. Amphetamine or stimulant class compounds, even coffee, also possess the side effect of pupil dilation.
In fact, it's when your pupils DON'T dilate or constrict in response to a light stimulus that you're in trouble. Doctor's often test for a patient's brain death by shining a light in the eyes to see if the pupil dilation reflex is still working. If not, usually it's due to brain death.
...I've never heard of cats disliking this in humans, though.
