do your pupil dilate when you look at someone you love
In theory, yes. When we dislike someone, our pupils narrow on them and when we like someone, our pupils expand. Women manipulate this using eye make up as well as darkening the lights to set the 'mood'. In a darker room, pupils expand to let in more light and as a result it seems as though you like the people in it more. So I would say yes, if you are in love your pupils would generally dilate. However, do not use this to decide if someone loves you or not.
Given this principle of reciprocal liking, we now have a logical explanation to explain the attractiveness effects described above. When you look at a girl you're attracted to, your pupils dilate. Subconsciously, she notices your dilated pupils and concludes that you like her and are attracted to her. Since people who like her give her an ego boost and make her feel good about herself, she becomes more attracted to you than she might be otherwise.