What King Hrothgar actually do to Beowulf after the defeat of the monster Grendel is that he welcomed him into the Herot Hall and gave a speech to honor him and to celebrate his victory. Although it may be known that Grendel was a difficult monster to defeat, Beowulf bravely faced the battle leading to the defeat of Grendel. This is the primary reason why King Hrothgar cannot contain his happiness.
After giving the speech, King Hrothgar symbolically welcomed Beowulf as a member of their family. Not necessarily literally, but it meant that upon defeating the monster, he is now one of them in vision and in the heart.
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