The answer to this forms the basis of this entire story.  The answer can be found early in the book, on p. 18 in my copy.  There, Mafatu makes a vow to himself.

If he could win his way to a distant island, he could make a place for himself among strangers.  And he would never return to Hikueru until he should have proven himself.

So, Mafatu is planning not to return to Hikueru until he proves himself.

But what does he have to prove?  This is the basis for the story.  Mafatu is afraid.  He is, specifically, afraid of the ocean.  The ocean killed his mother, and almost killed him, when he was a little child.  Because of his fear, Mafatu is something of an outcast.  The other boys do not respect him.  On p. 16 and 17, Mafatu hears various boys talking about how they will be going out fishing the next day and about how he, Mafatu, is the only one who is too scared to participate.  When he hears this, he makes up his mind to leave the island and not return until he can prove himself.

Starting with p. 18, the rest of the book is about what Mafatu does when he goes off to try to prove that he is not a coward.