In Doris Lessing's short story "Through the Tunnel," Jerry keeps his swimming accomplishment to himself perhaps because he doesn't want his mother to know what he has been doing (so she doesn't scold him or worry) and perhaps also because this feat has been a personal achievement that he wants to reflect on for a while.
First, Jerry definitely has a sense that if his mother knew what he has been doing, she would stop him. After Jerry spends time with the local boys and realizes that they are swimming through an underwater tunnel, he decides that he must do it, too. He cries at first because he can't and because the boys seem to reject him for it. But then, the idea takes hold of him, and he actually begins a training regime that is unusual for an eleven-year-old boy. He builds up his ability to hold his breath underwater, working at it gradually over several days. Yet he never tells his mother about it and downplays the problem of the bloody nose he gets at night. One day, his mother insists that Jerry stay with her, and this drives home the point that he should not tell her. After all, Jerry's mother worries about him. He is an only child, and his father is dead.
Jerry doesn't even tell his mother after the fact, for again, he realizes that she wouldn't like what he has done. She may not allow him any future freedom (for she has struggled enough with letting him go on his own this time around). At the very least, she would probably give him a sharp scolding for taking such a risk, and she would worry all the more about him next time. Jerry does seem to love his mother, so he would want to avoid that.
Further, Jerry wants to keep his accomplishment to himself so that he can reflect on it for a while. He has done something major here by swimming through that tunnel, yet it has scared him a bit, too. He realizes both his new-found strength and the danger in which he has placed himself, and he needs some time to process both. He must meditate on his reaction to his achievement (which is likely not quite what he has expected) and decide what it might mean for him in the long run; and this is something no one else, not even his mother, can tell him.