John Steinbeck began writing his final novel, The Acts of King Arthur and His Noble Knights, in 1956. The novel is an adaptation of Arthurian legend, based on the Winchester Manuscript text of Sir Thomas Malory's Le Morte d ‘Arthur. As such, Steinbeck presented the story with a somewhat more contemporary style while remaining true to the original tales.
I believe the scene you are asking about occurs when Sir Lancelot and Queen Guinevere meet by chance and share a surreptitious, yet passionate, embrace. Guinevere hastily departs to tend to King Arthur, leaving Lancelot weeping beneath the staircase. First, and most obvious, Lancelot is heartbroken over the loss of his true love, Guinevere. Although he and Guinevere most definitely love one another, he understands that their love must remain unfulfilled. Lancelot has pledged his undying loyalty to King Arthur, and, as he is an honorable knight, he cannot bring disgrace upon himself or break his moral code by betraying the king. Lastly, as an Arthurian Knight, Lancelot is often in the public eye which, again, means he must comport himself with integrity befitting a knight.