First, one must consider how removed Lanval is from his society at the very beginning. Even though he exemplifies chivalrous values, such as loyalty to his lord, generosity, and valor, he is not accepted. The virtues stressed in the lais are loyalty and justice. In terms of the former, Lanval is praised for having been true to Arthur even as Arthur shows a lapse in duty to him. Arthur, in forgetting to grant Lanval land and wealth, is failing in his own feudal duties, in which the lord also owes the vassal loyalty. As an individual from another land, much like Marie herself, Lanval is a man removed from the world. Loyalty is not rewarded in this world, where the cruel lies of the insulted queen can force a trial. It's no wonder that the lady asks that Lanval keep their love secret: such beauty, known only to two lovers, has no place in the ugly world.
Therefore, his greatest love must also exist separate from the Arthurian world around him. Marie rarely spends such narrative time in the descriptions she accords both the beloved lady and the lady's adornments, which suggests that we are to recognize how otherworldly they are. Her most common rhetorical technique is to praise what the lady has as superior to anything that has been known before. The sense seems to be that we should identify this lady as magical.
There are several elements and symbols that help characterize this idea that Lanval's love is meant to exist separate from society. Firstly, he leaves his horse behind to visit the lady. As a horse is always associated with a knight, this implies that he is willing to forsake the world that makes him an outcast. And he is well-rewarded by the splendid tent and gorgeous woman, whose only request is that he keep their love secret. Lanval takes her wish to heart, and only intensifies his devotion to solitary living, so much so that when Gawain invites him out to take leisure with the other knights, Lanval's first instinct is to go off by himself
Ultimately, the two are reunited in a happy ending, but it can't take place in the world. Instead, they take off for Avalon, a mystical realm, and he is never again seen. The world's inadequacy is stressed by the lady's refusal to stay in even the finest of Arthur's rooms, which has been ornately decorated specifically for her. Even the best the world has to offer is inadequate. Marie wishes us to realize the tragedy that such an intensely personal love will always be at odds with the world so dedicated to society of others. In a way, the lais exists as a warning against giving too much credence to our fantasies at the expense of the world around us.