As we know, spin is an internal symmetry. but it seems a bit different from other internal symmetry, e.g. electrical charge, color, flavor... because it can be coupled with orbital angular momentum and in some aspects be linked to space-time. further more, we can classify all the particles into two families, bosons and fermions, which seems to make spin more important than any other internal symmetry. I am a bit confused of this, why is it special from others?