There are many different theories as to why when you get married you wear the wedding band on your left ring finger.  For instance, the Roman belief is that the wedding band is worn on the left ring finger because of a vein in that finger that is called the vein amoris which is believed to be the vein that runs to the heart.

The Chinese explanation goes like this. Look at your hand. 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 your hands together and bend your middle finger down like this....

Now pull apart your thumbs, this represents your parents which will one day eventually not be apart of your life.  You are not destined to be with your parents forever. Repeat the same notion with the index or pointer finger which represent your siblings because like your parents, you are not meant to be with your siblings forever.  You will go on to lead separate lives.  And then pull apart your pinkies representing your future children.  Your children will one day go on to get married and lead separate lives as well. 

Now try and separate your ring finger that represents the person you married or life partner. Can't do it can you? All because you are meant to be with that person the rest of your life.

How cool is that!