The speaker remembers a boy who seemed to have everything, yet carried a hidden sadness. When asked what lay behind his sighs, he answered simply: "my plain brown eyes." The speaker laughs at first, then starts to worry. How can someone so lovely believe his eyes are the reason his happiness sinks?
So the speaker answers with honesty. Claiming never to lie, they tell him they adore his eyes, that blue eyes aren't stunning or unique, and that they would happily trade for his. The compliment is warm and a little clumsy, which makes it feel sincere rather than polished.
The ending turns into a three-part ode. The boy wishes for change, but the speaker hopes he will stay exactly the same. The mood is gentle, tender and quietly hopeful. It reads like a friend's reassurance, written to remind someone that what he dislikes about himself is the very thing another person treasures.