This has always bugged me, but it appears that the answer is out there via this: "But deep theoretical reasons having to do with the rotational symmetry of nature lead to the existence of spins for elementary objects and to their quantization." The sentence before says this: "A simple answer might be, perhaps they are composite, too." Which was always what I had assumed, simply because it seems to make sense (not just for explaining spin). Can someone link me to these "deep theoretical reasons"? I may not understand them for a while, but at least I'll know what questions I need to be asking, and that's a pretty good start in my book. Thanks.