In Beowulf's final battle against the dragon, Beowulf shows his true heroism and true strength, in spite of the different reasons for the fight. In his first two recorded conflicts, against Grendel and Grendel's mother, Beowulf was acting as a savior, swooping in and conquering the beasts that have tormented Hrothgar and his people. But in the final battle, Beowulf's motivations are far different.

Beowulf admits openly that this final battle is about his own strength and the wealth he can gain. He freely admits that this battle is a selfish one, and for that, he wishes that none of his men come with him, because he doesn't want them to perish on a selfish journey. Unlike the other battles, however, Beowulf's men actually rally around him and remain alongside him to help him conquer the dragon, showing that he truly has become legendary.

The final battle with the dragon is different for several reasons.  First, the motivations for fighting the dragon are different from that fo Grendel and his mother.  The original reason for Beowulf to fight Grendel and mother was to provide service to Hrothgar, because Grendel kept attacking his people and being a nuisance. 

Beowulf fights the dragon for different reasons.  Instead of doing it as a service for someone else or to protect people from danger, Beowulf elects to battle the dragon for selfish reasons, telling his men:

"This is not your fight, nor is it fitting for any but me alone to test my might against this monster here and achieve heroism. I shall win that wealth mightily." (XXXV)

Another factor that influences Beowulf's defeat that did not occur in the fight with Grendel and his mother is the abandonment of Beowulf by his men.  All "ran off to the woods to save their lives..but one" (XXXV)  When he fights Grendel's mother in the lake, Beowulf is alone, but his men stayed on the shore  and "stared at the waves; sick at heart, they wished and yet did not expect to see their winsome lord again" (XXIII)

Moreover, Beowulf's noble sword, Naegling, "was splintered; Beowulf's blade, though ancient and gray, was broken in battle" (XXXVI).  The dragon's fire had already burned the shield "down to the boss," so when the dragon assaults Beowulf again, he is without a weapon to defend himself.