What were the narrators first thoughts when he saw and heard the solitary reaper?

The speaker in "The Solitary Reaper" is enchanted by the woman who is singing as she works in the field.
The speaker's first words to his audience are "Behold her." He invites others to see the woman, but also cautions if they're not interested, they should just "gently pass" by without disturbing her. Thus, the speaker takes on a protective role as he delights in the song the woman sings; he wishes for no one to disturb her and possibly stop her singing. The speaker states that her sad song is so beautiful and clear that the valley is "overflowing with the sound." So captivated is he with her voice that he stops and again asks the audience to "listen!" Note Wordsworth's use of exclamation points in the first stanza to indicate the speaker's excitement with what he hears.
In the second stanza, the speaker compares the woman's voice to a nightingale's but feels the woman is a much better singer. He continues to praise the singer's voice as even better than that of the Cuckoo bird, no matter where one might travel. "A voice so thrilling ne'er was heard."
The speaker admits in stanza three that he cannot understand the words of the song, but he guesses some possibilities. Perhaps the melancholy woman is singing about battles, or some loss she has suffered through. No matter the subject, the song—as well as the voice—continues to charm him.
Wordsworth concludes his poem with the speaker suggesting the singer will continue singing "As if her song could have no ending," even after he moves on. The woman's song has had a profound effect on the speaker, and he admits he will carry it in his heart "Long after it was heard no more."