Why would a 15 year old girl's eyes look dilated all the time?

Dilated pupils: The most common reason for dilated pupils is drug use. There are certain drugs that cause the pupils to dilate, and many of these are abused by teens. Alternatively, it's becoming more popular in teen girls to use actual dilating drops in their eyes, as wide pupils are considered attractive to many of the girls and they do it to bring attention to their eyes. Have seen more abuse of these drops.

Drugs: Would be the first concern.. Have her see her doctor asap. She could also be using eye drops that do it or have a benign condition where the pupils are dilated. However, your question indicates it does not seem normal so i would have her seen by your regular doctor and an ophthalmologist.