In The King in the Tree, what is the main reason for the king's trouble?

The third novella in Millhauser's collection, the eponymous "The King in the Tree" is a reworking of the old Tristan and Isolde legend. The king in question is the cuckold King Mark, whose comely young wife Isolde conducts a passionate affair of the heart with the Cornish knight, Tristan. This illicit tryst represents a serious breach of Mark's trust as he had entrusted Tristan—who is, after all, his nephew—with taking care of his bride.
The story is told by Thomas of Cornwall, King Mark's loyal adviser. According to him, rumors of Tristan and Isolde's affair are rife at court, with many courtiers openly demanding that the king exact a bloody revenge on the amorous pair. But King Mark prefers to turn a blind eye to what's been going on right under his nose. Through an epic feat of self-delusion, he's managed to convince himself that all is well.
To prove Tristan and Isolde's virtue he hides in a tree near one of their regular meeting places to spy upon them. Although he finds to his satisfaction that there's been nothing untoward going on after all, Mark's mental turmoil is only increased further. If Tristan and Isolde have not consummated their love, he thinks, then perhaps that means that theirs is a higher love, a spiritual love, a love that Mark himself will never be able to reach. The king's biggest problem is his evident insecurity. Whatever the true state of Tristan and Isolde's relationship, he will never truly be at peace with himself.