Why does Beowulf leave this king to help out Hrothgar?

Beowulf's father Ecgtheow was a good friend of Hrothgar's. Long ago, Hrothergar saved Ecgtheow's life. Beowulf comes to repay his father's "debt" to Hrothgar. This is the 'technical' reason he helps Hrothgar. Really though, Beowulf is a true hero. He seeks out a challenge, and Grendel is the ultimate challenge.