If we examine the poem carefully we can see that the fairy-lady that the knight meets and is so taken by is responsible for deceiving the poor, unsuspecting knight by clearly leading him on and pretending to have more affection and love for him than she actually feels. Note what the following stanza reveals about her behaviour towards the knight:
She loooked at me as she did love,
And made sweet moan.
This is one example of the way in which the lady gave the knight expectations of her love and desire for him. Note the way that there is almost a sexual connotation in "made sweet moan" which, through its onomatopoeia, seems to capture the sexual desire and frisson between the pair. Of course, as the rest of the poem shows, this is just a deception designed to entrap the knight in the lady's snare, which is evident by the fact that the knight is still wandering around, suffering from unrequited love, when nature itself is abandoning the scene.