Low light can cause pupil dilation as they get more open to let more light in helping you see better. Also in extreme situations like life or death they let dilate so you can take in more information around you. If you see something you like ( Or someone you loooove :D ) It can cause them to dilate as well. Some say you can control pupil dilation by thinking of a special someone, or purposefully getting your adrenaline going. I have tried this by looking in a mirror and depending on thoughts I have, I can see a change in it. I also noticed by focusing on my eyes in the mirror, it causes my eyes to dilate :)
I had a cross-eyed teacher who could occasionally control her pupils.