Commonly, people won't get dilated eyes when they see something they want. But their eyes may lighter when they see something interesting. But there are many factors that can contribute to dilated eyes. Medicine such as Amphetamines, Atropine and Cocaine can cause dilated eyes. Besides, other factors such as brain tumor, head trauma or stroke are considered the causes of dilated eyes.
As to me, my eyes will dilate when I see something I want. My friends always say that. When we want to see something clearly, the pupil of our eyes will dilate too. But I also found that many people do not show their likes and dislikes when the things happen to their lives. I don't know why. But to those whose eyes will dilate when they see what they like, they are easier to get along with.