My thoughts on why synchronicities happen, is that in one way or another, we're all connected to the source or, certainly, we are that source. That source includes everything, this universe, all the universes if there were to be more than this, and whatever we're not aware of just yet. The why of we are not aware of this, or the very few in human history who have been aware of this have been called the enlightened, —Jesus in Christianity, Buddha (or Siddharta Gautama) in Hinduism— and it is not due to psychic abilities or chosen by any gods, these have been people who have reached human individuation, speaking in Jungian terms. Perhaps the way of individuation, one must have played all roles, the evil, the teacher, the 9-5 worker, the drug addict, to have been through the underground, the roots of the tree, the hell so as to say, to later on reach up high to heaven. A soul, consciousness, or however one wants to name it, when it has gone through all this and has gotten tired of playing the persona, —the masks we wear— how society tells us who we are or have to be, of sensing we're ego trapped in this body, once one is aware of such and works its way to his truth, which means the way to find it is unique, and a method which may have worked for Buddha may be different for John, or Brittany, or Bjørn, or Peter, or Laura, etc.
All this written previously is the context to now give way to why synchronicities may happen. If individuation means the way to the self, to break from the masks we wear, from the persona, perhaps when we are on our way through individuation, the source communicates through a synchronicity. That doesn't mean a synchronicity is something that only happens to few people who are on the path of individuation, it happens to all of us, yet is the one who is aware of such, gains insights and clues towards his truth.