Donatello’s already lost his oldest brother and his father; he won’t lose anyone else. Even if that means keeping them confined to the base or locked in his lab, your safety is his top priority, even from yourself, if it comes to it. He’ll never lose another loved one again, and he’ll kill anyone that try’s to take his family from him. You're too important for him to trust your safety to anyone else, so he’ll keep you by his side to ensure you remain safe and healthy.