Beowulf is the hero who embodies the religious faith and the qualities that his people admire--bravery, loyalty, honor, strength, integrity, determination to do the right thing, and a will to protect the weak.
It was always understood that such a man would someday be the leader and King of the Geats, Beowulf's people. Of course, he is born or noble blood, since the King of the Geats at the time of Beowulf's travels to rescue the Danes from Grendel is Hygelac, Beowulf's uncle. As Beowulf spends his youth gaining experience and fame through his travels and successes against such monsters, it is literally in his blood to return home and some day be named as the King.
Furthermore, he does not just sit on his laurels upon the throne, but he dies a valiant death fighting a dragon which threatens the safety and well-being of his kingdom. Upon his death, he names his own cousin, Wiglaf, to be King.