After the Finch family comes back after the fire and Atticus asks Scout where she got the blanket, why did Jem start rambling off all their secrets in Chapter 8 of To Kill a Mockingbird?
Like about what they had found in the tree and how Boo had never hurt them and about how they couldn't just give the blanket back. PLEASE HELP!!
It was probably a combination of guilt and sympathy for Boo that touched off Jem's ramblings on the night of Miss Maudie's house fire. Jem had already come to the realization that Boo was not a danger to the children and that he wanted to be their friend. Jem also felt guilty about the night raid they had made on the Radley house when he had lost his pants. He went back to retrieve them in the first place because he didn't want Atticus to find out that Dill was lying when he claimed to have won them playing "strip poker." Atticus trusted Jem, and Jem wanted to keep it that way. So, when Atticus explained that "all of Maycomb was out tonight"--meaning Boo, too--Jem quickly understood from where the blanket had come. It was proof to Jem that Boo had only good intentions, and he wanted Atticus to know that the children no longer believed Boo to be an evil ghoul, as they once had.
Jem's response results from a number of important causes. There are elements of guilt, empathy and a budding maturity which come together rather suddenly, causing him to blurt out so much information, much of it personally incriminating, all at once.
First, Jem is basically a good kid. He is family oriented and generally obedient, though with a thirst for adventure that sometimes gets him into trouble. We see his love for his sister demonstrated when he pulls Scout out of trouble at school. His self control is demonstrated when, despite great provocation, he ignores the insults and imprecations of Miss Dubose (until she insults his mother, then all bets are off).
The thirst for adventure crops up in the games he plays with Scout and Dill; the tire rolling game, "playing" Boo Radley and finally, the after dark raid on the Radley garden. But his guilt accompanies the adventure. He knows, when the three friends begin their forays into Boo Radley's yard, that they are invading another's privacy and that it is wrong to do so. Further, he must at least fear that he is leading his sister into what might be a dangerous situation. We see his perception of the danger Boo Radley poses when he is afraid to run up onto Boo's porch (even though he does it anyway) early in the book and later, when he tries to discourage Scout from accompanying him and Dill into the Radley's back yard. We know that he feels guilty over these incursions because of the lengths he goes to in trying to hide them. Risking bullet wounds over a pair of pants simply because you can't hide the fact that you lost them is an act that must be guilt powered.
The predominant reason, though, that Jem spills his guts about all that has gone on, probably has a great deal more to do with compassion than with fear. By this point in the book, he has discovered the truth behind the myth of Boo Radley and has developed a deeper understanding of the character of the recluse. What will happen to Boo if the town learns that the man, whom they have long been used to seeing as a possibly dangerous but homebound recluse, actually leaves his home on occasion, and at night, no less? Boo's misunderstood past is a central topic in the scariest of town legends. If the townspeople find that he has left his house, come into their midst, and that during such a catastrophic moment, without being seen or heard, their reaction may be judgemental, possibly even violent. Whatever the consequences, Jem knows that Boo will be the one to suffer them.
Added to his guilt and compassion, Jem is growing up. It is true that the comparison between his own immature, nearly cruel, actions and those of a much maligned character whom he has found to be gentle and kind make him feel guilty. However, it is his own budding maturity which leads him to the confession that he hopes will protect Boo's privacy and perhaps his safety as well. Putting the needs of another ahead of your own, especially when it could get you into trouble, is a sign of maturity.