Wide Eyes

Because the nervous system controls the muscles of the irises, the response of the nervous system to different stimuli results in involuntary pupil dilation. Our brains like to look at attractive things, and find enlarged pupils attractive.  Conversely, our pupils contract when looking at someone we find unattractive.

This goes for anything visually pleasing:  a snowy mountain or a boiled lobster next to a bowl of drawn butter.

There is a whole study devoted to pupil size as an indicator of emotion, known as pupillometrics.