Why the Left Hand?

Well, in some parts of continental Europe it is and always has been the right. There doesn’t seem to be any particular reason that the Christians should have mostly kept this the same as the original. But one thought is; as the man, facing his bride, reaches straight out with his right hand he naturally touches her left. As she does his, as now, with more and more men wearing one also, when the rings are exchanged.

This is a modern practice begun mostly during the second world war, a consequence of increased numbers of men being separated from their loved ones and seeking a cheering reminder. This almost happened earlier in history, with the advent of the gemmal ring, alternatively spelled gimmal or gimmel. This was two or three decidedly ornamental links, usually with hands and hearts or knots, fastened together by a hinge, or interlocking like the Olympic rings, and being capable of joining into one. At betrothal, they would be separated, with one given to the woman, one kept by her lover, and if present, the third held by a witness until the wedding day when all would be reunited and henceforth kept by the bride.
Back to fingers though, and the thumb briefly challenged the accepted norm in Elizabethan days as fashionable ladies deemed to wear their wedding rings there, but this did not last and so today it is as it was in the beginning, just like a circle really, or a ring.