I know that matter and corresponding antimatter particles annihilate giving energy and, conversely a photon can give a pair of particle / antiparticle. Perhaps this is a candid question, but is it only an empiric finding? or is there other reason? In fact the two particles of the pair have equal mass, different sign for charge... Mass does not annihilate mass and two particles of different charge or different spin do not annihilate either. I see the fact but I do not see the reason. Perhaps this is a basic fat not reducible to other but I would ask your opinion about it.