What hurts Mafatu most at the beginning of the story is that people think of him as a coward. In a warrior society such as this one, that's a particularly outrageous insult indeed. The last thing that anyone, especially a young boy on the cusp of manhood, wants is to be called a coward. Mafatu's name means "stout heart"—which, under the circumstances, seems like a cruel joke. Mafatu's very name hangs over his head, acting as a constant reminder that he has a lot to live up to.
Yet Mafatu can't get over the fact that he's scared stiff of the sea. He has good reason to be, as he was with his mother when she was tragically swept out to sea by a giant wave in the middle of a hurricane. But that cuts no ice with anyone in this society. Young men are expected to be brave, no matter what.
Whereas today we would tend to be more sympathetic to someone in Mafatu's position—his condition shows all the signs of Post-Traumatic Stress Disorder—in Mafatu's society it's different. Brave warriors are not just expected to stand firm against their rivals, but even against the spirits whom they believe to be responsible for causing adverse weather conditions.