Why did Beowulf become a hero?
In the Scandinavian society into which Beowulf is born, a son is expected to follow in his father's footsteps. In this case, Beowulf's father, Ecgtheow, is a great Geat chieftain. As Beowulf describes him to Hrothgar's coast-guard:
My father was well known to folk,/a noble chieftan by the name of Ecgtheow;/ he tarried many winters, ere he turned on his way,/old in years; any wise man/ readily remembers him around the Earth. (ll. 262-266)
If Beowulf behaves as his society expects, therefore, he is born to be a hero, a leader, a son who will reflect all the virtues of his father and family. In addition, Beowulf is the nephew of Hygelac, one of the Geats most powerful kings, and as part of Hygelac's immediate family, Beowulf would be expected to embody the heroic virtues of both his father Ecgtheow, who married Hygelac's sister, but also those leadership qualities directly attributable to Hygelac himself. In Scandinavian and, later, Anglo-Saxon societies, the bond between uncle and nephew is as strong as that between father and son.
In Scandinavian and Anglo-Saxon societies, the line of succession among kings and other leaders is from father to son. If that line is broken by a son who fails to exhibit the necessary physical strength and appropriate heroic virtues, the society is at risk because there is then a gap in leadership that creates chaos.
If we were to perform some psychoanalysis on Beowulf, then, we would conclude that Beowulf--as the son and nephew of great leaders and warriors--is born to be a hero. Because family is all-important in these societies, in a sense, Beowulf has no choice but to become a hero. To become anything less, in this society, would be disastrous for both Beowulf as an individual and the society as a whole.