Why is Jem so upset the knothole is plugged in To Kill a Mockingbird?

Jem and Scout have received little mysterious treasures such as
gum, a spelling medal, and small carved figures in the knothole of the tree, and Jem is fairly
sure that these items were placed there by the mysterious Boo Radley.  The children have been
facinated with the man since the start of the novel, and they are slowly learning a little bit
about him from the things he is leaving for them.  Once Boo's brother fills in the knothole Jem
knows that what little contact the children had with Boo is cut off.  Jem is old enough to
consider the implication of the gifts in the tree and what they say about the man who gave them. 
The gum would quite a nice treat for children; the spelling medal would have been something that
a young Boo Radley took great pride in and it reveals that he was a smart young man; the carved
figures that look at lot like Jem and Scout reveals that Boo Radley is paying attention to the
children as they run and play in the neighborhood.  The reader and the children are realizing
that there is a lot more to Boo than the "folk-tales" told about him.