Here's why Ulrich cries out in joy:
* He sees the futility of their feud: The shared danger of being trapped under the tree makes Ulrich realize the pointlessness of their long-standing animosity. He sees that their past conflicts are insignificant compared to their present shared situation.
* He recognizes their common humanity: The shared experience of fear and potential death brings them closer. He sees Georg not as an enemy but as a fellow human being struggling for survival.
* He experiences a sense of relief: The realization that their feud is irrelevant in the face of their shared fate brings him a sense of relief. The tension and animosity he carried for years have been lifted.
* He sees a chance for reconciliation: The moment offers an opportunity for reconciliation and a chance to build a new relationship with Georg.
His cry of joy is a release of the bitterness he's carried for years and a hopeful expression of the possibility of newfound unity and peace. However, the story ends on a bittersweet note, as they are discovered by their respective hunting parties and their fate remains uncertain.