When Jem goes to touch the Radley porch, he accidentally gets his pants stuck and has to leave them there. The children make up a story about playing poker with matches, but they know that in the morning Nathan Radley, who claims to have shot at someone with a shotgun, will find the pants.
Scout tells Jem that when Atticus finds out about the pants it will “be pretty bad.” She thinks that Jem will be whipped by Atticus. However, Jem does not want this to happen.
He blew out his breath patiently. "I- it's like this, Scout," he muttered. "Atticus ain't ever whipped me since I can remember. I wanta keep it that way." (ch 6)
Jem is getting older when this incident with the pants happens. It is one of the indications that Jem and Scout are growing apart as Jem matures. Scout does not understand why Jem would risk his life, as she sees it, to retrieve his pants.
She does not understand the for Jem, the bigger problem is not Atticus punishing him, it is Atticus being disappointed in him. At this point in his life, Jem wants to be seen as making more adult choices. He wants to be seen as someone to be trusted. He knows that if Atticus finds out about the pants, he will know they are still going after Boo Radley. Since Atticus wants them to stop, he does not want Atticus to know.
The pants are a turning point in Jem and Scout’s relationship. When he goes out to get his pants, Jem is actually making a grown-up decision. It is not what he does that matters, but the reason he does it.
Atticus trusts Jem, and Jem doesnt want to loose his trust. Therefore he goes and looks for his pants.