Why did Tavana Nui's silence make Mafatu feel ashamed?

Tavana Nui is the Great Chief of the Hikueru, and Mafatu’s father. His silence shames Mafatu, because the boy knows that it signifies his disappointment in his cowardice. Mafatu knows that his father loves him and is understanding of his fear of the sea. However, the Hikueru have roles spelled out for each of the genders, and it is expected that boys should grow up into strong warriors and fishermen, able to protect and fend for the larger society—roles which Mafatu is unable to take up because of his fear. The villagers talk about Mafatu’s lack of bravery, some even taunt him. They ask themselves questions like "What manner of fisherman will Mafatu grow up to be? How will he ever lead the men in battle against warriors of other lands?" The text states that the Great Chief would grow "silent and grim" upon hearing this kind of talk from the villagers.
Being the chief’s son places a lot of responsibility on Mafatu’s young shoulders. The older people of the tribe quite understand Mafatu’s fear for the sea and blame "tupaupau, the ghost spirit which possesses every child at birth," for it. They do not pressure him into overcoming his cowardice, perhaps because they also understand the genesis of the boy’s fear—the boy has never forgotten the hurricane that swept him and his mother into the ocean, when he was only three years of age, finally killing his mother and leaving him for the dead. In fact, he is haunted by the voice of the reef, whispering these words in his ears: "You cheated me once, Mafatu, but someday, someday, I will claim you." The younger people of the tribe mock Mafatu often. The remaining members of his family also treat him with "open scorn." His father, on the hand, is silent—a silence that shames the boy. This is one of the reasons why Mafatu finally decides to leave the island to face his greatest fears and to prove to all, his bravery. He hopes to come back to Hikueru a brave man, deserving of the name “Stout Heart,” as given to him by his father. He hopes to make his father a proud father of a reformed, braver him.