Symbolism, Imagery, Allegory
When Tristan gets all emo and starts lamenting all he is suffering because of his love for Yseut, he asks why love "wounds" him (18.152). He also complains that he is in "great agitation at every moment of the night and day" and going out of his mind because he cannot see Yseut (18.153). Tristan decides to put on the disguise of a "fool" or madman, so that his exterior costume matches the way he feels on the inside.
So what that means is that by pretending to be crazy, Tristan is actually showing everybody what he really feels like inside, because love sure is driving him off the deep end. And our suffering hero sure knows that only a trip to Dr. Love—er, Dr. Yseut can cure that achy breaky heart: after all, when he finally reaches Cornwall, he tells Brangain that only Yseut can "cure" this sickness, just as she previously cured him of his physical ailments.
So in this way, Yseut's role as a healer is a mark of her role in the love-relationship just as Tristan's disguise as a madman is a mark of his. The idea of love as a wound, disease, or madness that only the female beloved can cure is a very common one in French courtly poetry of this time period. It emphasizes the physical effects of all-consuming love upon the lover. But seriously, it's not like love feels that much different today, right? Love hurts! Don't we just want somebody to make it better?