Since you're asking for an opinion, and not a factual answer, this question really should be on the discussion board.
Having said that, I'll give you my opinion. Beowulf goes to help Hrothgar for one reason: fame. He wants to make a name for himself. That's why he fights Grendel on his own; he'd have to share the spotlight with someone else if he let his men help him. In Norse mythology, in order to get into Valhalla, "the Hall of Heroes" (the Viking heaven), not only did you have to die in combat, but you also had to be the bravest warrior. Certainly facing a monster alone would qualify Beowulf to be the bravest warrior. If he defeats Grendel, he will be famous among all men. If he is killed by Grendel, he will be assured of going to Valhalla. It is a no-lose situation!