What does the dragon become angry during its discourse with Grendel?

In Chapter Five, Grendel meets the dragon, an ancient, red-gold, gigantic creature resting upon and guarding its hoard of golden treasures. The dragon mocks Grendel, claiming to know his thoughts—in fact, to know everything that ever has happened or will happen. It attempts to engage Grendel in deep philosophical discourse regarding the nature of reality, but is frustrated by Grendel’s inability to comprehend.