Unrequited love is love is that is not reciprocated, even though reciprocation is desired. This can lead to feelings such as depression, anxiety, and mood swings such as swift changes between depression and euphoria. The experience of being in unrequited love is often one of limerence.
Unrequited love can result in obsessive behavior or transform into hostility to the object of desire, leading to behavior such as stalking. However, it has also been the inspiration for and topic of many great works of art.
Although being in unrequited love can be intensely stressful and unpleasant, it can simultaneously be a source of great joy, and even provide the lover a sense of satisfaction at having somebody to love, even though that love is not returned. The emotional duress may be seen as "worth it".
Although unrequited love can last a very long time -- many years, or even decades -- the lover's feelings usually reach a breaking point as they continue to deepen. The love typically ends when the lover receives reciprocation from the loved (consummation), develops less intense feelings for the loved (starvation), or channels his feelings towards another, more reciprocative, object (transformation).
In Literature
One of the first classic examples of unrequited love in literature is the romance between Don Quixote and Dulcinea in Miguel de Cervantes' Don Quixote. Don Quixote, who believes he is a knight, imagines that he serves a noblewoman named Dulcinea. Unfortunately, her name is already taken by a peasant in his hometown, and his love for her is not returned.
See also