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.