Jem is struggling with several forces at work in his life here. First off, he was getting pretty excited about the newly formed relationship the kids were having with the giver of the gifts. This was especially significant to Jem when the giver hand carved soap dolls to be spitting images of the kids. I think this made Jem feel pretty special to be so specifically focused on. Secondly, Jem is getting older. He used details to uncover a truth about all adults: they lie. This becomes important much later, but here Jem caught Mr. Nathan Radley lying to him. Atticus defends the lie, but it doesn't seem to sit well with Jem and a significant growing pain occurs for Jem. Jem also looks up to his dad. To think it is possible his dad is covering something up may be significantly effecting him too.
An even simpler answer is to look at Jem's changing age. Getting on in years, Jem is 11 or 12 and beginning puberty. No matter what the gender of an individual, those are emotional years.
My interpretation was that he had just understood something very important about boo. The children mocked boo radley in their youth thinking nothing of it. The detachment of boo from society, his dehumanizing nickname and the rumors circulating society made him seem not entirely real, as if he was a sort of ghost haunting the radley house ("inside the house lived a malevolent phantom") therefore making him easy to mock and more difficult to sympathize with. After the children had begun finding items in the tree, it was not only exciting, it clarified his existance and contradicted the idea of him being feral and uncaring. but it was not until the knothole is filled with cement that the full weight of boo's situation hits Jem. Giving presents to children at a distance is such a simple and harmless thing whatsmore, it was probably the first contact boo had with the outside world in years but it was cemented over by Mr. Radley and jem realized just how trapped and lonely boo really is. It is sad to think of any human being confined to a room as boo was but it was never recognized until the presents made jem see boo as a human being.