The theme of this poem is about the love for a woman that he never pursued. He speaks highly of this woman for her beauty, and how she stood out from all others. He also never really speaks about ever being with her or even talking to her. In the last four lines he speaks of her death and how she was unknown to many. This leads to believe that no one would miss her as much as he would. I do not think it is so much lust as it was admiration for this unrecognized beauty, so he admired a beautiful woman that he never actually pursues until it was too late.
The theme sets the mood for whom and what he is writing about when reading the poem. If this theme is taken into perspective while reading it; the reader can see this secret admirer. The reader, per say, could imagine a man who passes by daily seeing this poor maid or farm girl, who know one really acknowledges, and sees the beauty in her. But before he can pursue her it is too late, she has already passed away.