Why is the wedding ring worn on the fourth finger on the left hand?

Roman Belief:
According to tradition in some countries, the wedding ring is worn on the left ring finger because the vein in the left ring finger, referred to as the vena amoris  was believed to be directly connected to the heart which we all know as a symbol of love.
The wedding ring

Chinese Belief:
Look at your left hand, the hand that your wedding ring goes on, and do you see your thumb? Well the thumb represents your parents. Now your index finger, or your first finger, represents your siblings. The third finger, loving called the middle finger by some, represents yourself. Where the fourth finger represents your life partner. And your pinkie is to represent the children you will some day have.
Now place both of your hands together and bend your middle fingers together, and leave the other fingers touching together.
When your hands are as such do this :
Pull your thumbs apart. (this represents the parents) Your thumbs will open because you and your parents are not destined to be together forever. At some point they will leave you in your life. Now put your thumbs back together.
Repeat the same motion with your index fingers (this representing your siblings). Of course your index fingers will also separate because you are also not destined to be with your siblings for the rest of your life. They will move on and have families of their own. Now put them back together.
Now separate your pinkies. (representing your future children) Of course these will also separate because your children will not be with you forever. They will grow up to have lives and families of their own as well. Now put them back together.
Now for your "ring" finger. (representing your life partner) Try separating them as you did with the other. Can't do it can you? That is because your life partner is the one who is supposed to be with you for the rest of your life. They will be the one to be with you forever and through thick and thin.

Wedding rings