What enrages Grendel when he first arrives at "the grounds of Hrothgar's dwelling" on the night of his battle with Beowulf?

The happiness, the joyful atmosphere of which he was not a part of, enrages Grendel.

Explanation:

The epic poem "Beowulf" tells the heroic story of Beowulf and his fights against the monster Grendel, his mother, and later on a dragon. The bravery and courage that Beowulf exhibits in his fights show the greatness of the person.

Grendel had always tormented, ravished, and ate the men he managed to kill every time he went to the Geats' hall. He knows they will be assembled inside the hall, drinking and enjoying which presents a good chance for him to attack and torture them. On the night of his battle with Beowulf, Grendel was extra angry because he was "Spurned and joyless". He hates the men enjoying while he was left alone in the swamps.  

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