In chapter 3, why does Ralph reproach Jack?
In chapter 3, Jack returns from tracking a pig and rudely demands water from Ralph, who is struggling to build a shelter with only Simon's help. Ralph is upset at the other boys for not helping build shelters. All the boys helped with the first shelter, but only three boys helped Ralph build the second, and now only Simon is helping with the third, and it's not going well. Ralph complains about the other boys to Jack, and Jack begins to get defensive because he and his hunters were excused from building so they could get meat. Yet they have not successfully killed a pig. When Jack starts to explain his hunting effort, Ralph lashes out at him. Jack says, "We want meat—" and Ralph interrupts with audible antagonism, "And we don't get it." Ralph then tries to convey the importance of shelters to Jack, not just as protection from rain, but also to provide emotional security for the littluns, who are afraid at night.
Although Jack briefly acknowledges the importance of creating a "home," he soon gets caught up in his obsession with hunting again. As Ralph is discussing rescue, Jack thinks about where the pigs might be hiding. Ralph rebukes him, "I was talking about smoke! Don't you want to be rescued? All you can talk about is pig, pig, pig!" Ralph then says that he works hard all day and Jack comes back and doesn't "even notice the huts."
In short, Ralph reproaches Jack because he feels Jack has been shirking all other responsibilities, and encouraging his hunters to do the same, in order to pursue his obsession with hunting, which hasn't yet delivered results.
Ralph reproaches Jack because all Jack wants to do is kill a pig, but he hasn't been able to do it. Jack has become obsessed with the idea, but he has nothing to show for it. Ralph on the other hand has been trying to build shelters and needs help, but can't get anyone except Simon to help him.