Why does the song of the Solitary Reaper give immortal happiness to the poet “even it was heard no more”?

Q:  Why does the song of the Solitary Reaper give immortal happiness to the poet “even it was heard no more”?

Wordsworth was in the habit of having long solitary walks through the hills. Once he happens to pass through a distant valley in Scotland. He saw a lonely highland girl singing and reaping alone in the field. She was singing a' sad song which deeply impresses the poet. He remembers the song even after he has left the place. Then song of the Solitary Reaper impressed the poet so much for various reasons. 

The girl was singing in the natural beauty of the mountain in a valley. Her voice was enchanting. The poet says that even no nightingale can sing as beautifully as the girl was singing in a melodiouis voice. 

The whole valley was full of her sweet song. The poet says, though he could not understand what she was singing but her voice was fascinating and enchanting. He compares her note with that of the cuckoo's and says thaj her voice was sweeter than cuckoo’s. The poet says  whenever he is in a sad mood, he remembers that soft and feels happy. Although he did not happen to see the girl singing since very long, even then he remembers it. It is a source of pleasure for him.