The wedding ring goes on the fourth finger of the left hand because a special vein, the vena amoris or 'vein of love', runs from that finger directly to the heart. It is the romantic reason couples are told for the tradition, and it is repeated at weddings everywhere.
There is no special vein. The blood vessels in your ring finger are much the same as those in every other finger; nothing about that finger has a unique, direct line to the heart, and anatomically the 'vena amoris' simply does not exist. The idea traces back to ancient writers, possibly Egyptian or Roman, and was popularised in Europe centuries ago, and it is lovely, which is exactly why it endured. The real reasons the ring sits where it does are cultural and practical: tradition, religious ceremony wording, and the fact that the fourth finger is relatively protected and less used than the others, so a ring there is comfortable and stays out of the way. Different cultures even use different hands. So the heartstring running from your finger is anatomy fan fiction, a beautiful one that has outlived its own debunking. The custom is real and meaningful. The vein it is supposedly built on was never there.