To me, this whole book is (in addition to being an exciting adventure story) a story about Mowgli's attempt to find out where he belongs.
Early in the story, Mowgli's identity is that of a wolf. Everyone pretty much accepts that he is one of the wolves. But then, for example, he uses fire to defeat Shere Khan. Everyone knows that wolves don't use fire and this makes Mowgli and the others wonder what he really is -- what world he belongs to.
This theme continues as Mowgli is adopted by the woman who is actually his mother. He becomes part of human society, but only to an extent -- he clearly does not fully belong there. The people reject him, for example, when they think he is a sorceror because of his ability to talk to animals. So now we have Mowgli being rejected to some extent by both animals and humans.
Eventually, of course, Mowgli decides that he is going to have to be a human being. But this comes only after a long search in which Mowgli tries to figure out which world he belongs to. During this search, we are forced to ask ourselves what makes a person belong to some group. We have to think about whether it is physical characteristics, or behaviors and adherence to custom. These are issues that confront all people to some extent in their lives.