Final answer:
Mrs. Grose is distressed by Quint's relationship with Miles because of its negative influence on the young boy.
Step-by-step explanation:
Mrs. Grose is distressed by Quint's relationship with Miles because it represents an inappropriate and potentially harmful influence on the young boy. Quint, a servant at Bly Manor, has a questionable character and introduces Miles to immoral behavior and dangerous ideas. Mrs. Grose, who is responsible for the children's well-being, worries about the negative impact this relationship may have on Miles's upbringing and future.