Final answer:
Mrs. Phelps cries while hearing the poem 'Dover Beach' as it likely triggers a deep emotional response, potentially linked to her own personal losses or the awakening of repressed emotions.
Step-by-step explanation:
Mrs. Phelps cries upon hearing Montag read 'Dover Beach' because it stirs an emotional response within her, likely tied to a personal loss or hardship. The poem speaks to the loss of certainty and faith, provoking a response from Mrs. Phelps which could indicate that its themes resonate with her own experiences or stir an emotional awakening within her. None of the provided context offerings definitively connect to Mrs. Phelps' character or her reaction in the scene from Fahrenheit 451, so her exact motivation is open to interpretation based on the reader's understanding of the character and the novel's themes.