The idea of courtly love has been a recurring theme in Western literature for centuries, reaching its peak of popularity shortly after its invention in medieval times. It can be considered the chaste ideological predecessor to the relatively modern idea of romantic love, which, while not always chaste, nonetheless provided some kind of moral and ethical justification for sex, often within the legally and religiously sanctioned structure of marriage.
Although the idea of courtly love was reinvented by the poets who praised it and the practitioners whose life imitated the poets' art, the basic premise was as follows: passionate, obsessive, sexually unrequited love between a man and a lady (never a woman, always a lady — and almost always of noble birth). As Elizabeth Abbot wrote in A History of Celibacy:
...courtly love acknowledges human sexual longing but incorporates it into a great passion guided not by carnality but rather by the highest moral and aesthetic values.
Courtly love is an exalted state between a man and a superior woman he both respects and adores with quasi-religious fervor. Her love tests his resolve, firmness, and loyalty, for it is difficult to obtain. It is also immensely ennobling, so that his very suffering strengthens every aspect of his being: his military prowess, social standards, even his moral and religious perspectives. Sometimes, the mere thought of his beloved triggers these holistic improvements. (363)
I couldn't have summarized it better myself. The idea that love makes us better people comes out of courtly love, which ties the pain of unrequited love to the idea that suffering builds character. Courtly love was strongly linked to the rules of chivalry, some of which remain with us today. The Rules of Courtly Love, enumerated by Andreas Capellanus in the twelfth century, place additional restrictions on the nature and expression of courtly love, including jealousy and secrecy.
I'll close with an additional quote from Abbott:
Courtly love was agonizing and admirable, the source of chivalrous virtue. For these same reasons, it was often chaste, both because the logistics of consummation defeated the would-be lovers and also because, in some manifestations, courtly love was inherently pure....
Centuries of literature and lives imitating art transformed courtly love into romantic love, intense and unattainable, a phenomenon too high-mindedly impractical to survive marriage and the trials of time, routine, and old age. The precious instant of recognizing the beloved, the stylized pursuit, the exchange of extravagant words penned on scented paper, the self-indulgently obsessive meditating on each other — these became the characteristics of this new kind of love. Sexual attraction fueled it, but in this case as well, sex never dominated the lovers' agenda. (364-5)
Finally, as the chaste and secret nature of courtly love makes exclusivity irrelevant, I encourage others to add writeups to this, as mine is highly based on Abbott's book and it would be nice to get some primary source-based material here as well.