Ancient Anglo-Saxon culture operated largely on a merit-based system, so while nobility and lineage did matter, they did not have the final say in which individual became king. The next in line had to deserve the title, which meant that they had to show strength and bravery in battle, serve their king well, and care for their countrymen. There were no two individuals who were more loathed than the kin-slayer or the king-slayer in this culture, and this shows how much the Geats valued their familial connections as well as the social hierarchy they lived in.

By defeating Grendel and Grendel's mother (descendants of the most notable kin-slayer, Cain), Beowulf returns home to Geatland victorious. He has received gold and material rewards from King Hrothgar, and he would receive accolades from his uncle, Hygelac, back home. Due to his notable strength, size, and clear ability to command the men around him, as illustrated by his Danish exploits, Beowulf's ascension upon Hygelac's death is a given in the Geats' community. The final sections of the epic poem, which depict how the people mourned for Beowulf, illustrate the love and respect his people had for him. Before he fights the dragon, we are told, he enjoyed a long reign, expanded Geatland's borders, amassed a wealth of treasure for himself and his people, and treated his thanes with respect. His final act as king was to defend his people from the great serpent and name Wiglaf, a true and noble thane, as his successor.

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.

Chapters twenty-nine through thirty-one describe of Beowulf describe how Beowulf came to be the king of the Geats. Upon returning from the Danelands, Beowulf goes straight to his king, Hygelac, and presents his rewards from Hrothgar to him. Hygelac recognizes the greatness in Beowulf (given his renown, lack of cruelty, and possession of great strength). Knowing Beowulf's promise as a leader, Hygelac presented Beowulf with a noble sword, "seven thousand hides of land, with a large house and a seat of authority." This presentation proved Boewulf's worthiness to the throne and, essentially, served as Hygelac's naming of Beowulf as the next king.

Upon Hygelac's death (which came during a raid), the Danelands "came into Beowulf's hands."