Maybe she does not know how strongly you feel about long hair? Maybe she is tired of the upkeep required for long hair? Maybe she has a fantasy/fetish/enjoyment of having someone cut her hair? Just guessing here... I'd suggest to make your feelings known, and see what she is feeling, maybe there is some sort of compromise, like you do washing/brushing for her, she only trims her hair every so often, you get to play in her hair in exchange, etc. Maybe if she enjoys the cutting you can "play cut" instead and still satisfy her?