Why does my girlfriend cut herself?

This is an answer that only your girlfriend knows the answer to. You and I could try to theorize all we want too, but we won't truly know the reason. This is a conversation you should try to have with her, but remember that if she isn't comfortable sharing her reason with you, then you shouldn't press. If she is really struggling with a problem, then your girlfriend needs to see a trained therapist and get the help that she needs to get. You can be there for her by showing her support and compassion through this healing journey that she will hopefully go on.