check out this book by john gottman- i have posted on it before-it outlines really helpful ways to deal with your childs emotions and help them to identify how they are feeling and what they can do to feel better-its called 'the heart of parenting-raising an emotionally intelligent child'it also has a lot of ideas as to how parents can learn to 'coach' their children through different emotions so that the child learns about their emotions and how to identify what they are feeling and why.
your daughter is probably crying whenever she has feelings that she doesnt like, but doesnt know how to express in a constructive way, so she simply cries. she probably doesnt know any other way to deal with these emotions, but this book teaches you how to teach HER good, positive and constructive ways to deal with these emotions. good luck!