Arguably, the moral of "The Nightingale and the Rose" is that true love involves sacrifice and selflessness. Wilde makes this point through his characterization of the nightingale, the bird who gives her own life so the student can obtain a red rose. This selfless act is made all the more poignant by the attitude of the student, who has no idea of the nightingale's sacrifice.
The Student looked up from the grass, and listened, but he could not understand what the Nightingale was saying to him, for he only knew the things that are written down in books.
To reinforce his moral, Wilde contrasts true love with the love between the student and young girl. This form of love is based on materialism (the possession of a red rose) and is not long-lasting. As soon as the girl is offered jewels, for example, her attentions quickly turn to a new suitor and she loses all interest in the student. This prompts the student to turn his back on love and concern himself only with his studies.