What does Beowulf’s assistance to Hrothgar show about his character?

Hrothgar, King of the Danes, is a father figure to Beowulf and looks upon him as a son. He is renowned as a wise ruler, fair and just. In Beowulf's younger days he was something of a wayward upstart, in need of Hrothgar's firm, guiding hand. When he was a young king, Hrothgar settled a blood feud between Beowulf's father and his enemies. As well as sharing the respect of all nobles towards Hrothgar, Beowulf also has a deep personal regard for the old king.
So when the kingdom of the Danes comes under attack from Grendel, Beowulf does not hesitate to spring into action, setting off at once with fourteen carefully chosen companions to fight and destroy this terrifying monster. Not only does Beowulf show exemplary bravery, he also repays an enormous debt of gratitude he owes to Hrothgar. But he does not simply act out of duty. He performs the deed because he wants to, and because it is the right thing to do.
Beowulf's character has undergone quite a dramatic change since his wild, impetuous youth. He has learned well from Hrothgar; in addition to immense physical courage and a profound sense of duty, he has now achieved a degree of wisdom. We see this clearly illustrated when Grendel's mother attacks Heorot. Grief-stricken by the death of his loyal retainer, Hrothgar is crestfallen. But now it is Beowulf's turn to show wisdom in the face of adversity:
“Grieve not, O wise one! for each it is better,
His friend to avenge than with vehemence wail him..."
 
This is no time for tears; only vengeance. The great joy at Grendel's slaying has given way to even greater sorrow at his mother's terrible revenge. In urging calm upon the old man, Beowulf reminds Hrothgar of what he once said to him: that great joy often leads to great sorrow.