Here’s a rather convincing (and beautiful, depending on how you see it) explanation of why the wedding ring should be worn on the fourth finger.
The thumb represents your parents.
The index finger represents your siblings.
The middle finger represents yourself.
The fourth (ring) finger represents your life partner.
The last (little) finger represents your children.
Open your palms facing each other. Bend the middle fingers and hold them together, back to back. Stick the remaining three fingers and the thumb together from tip to tip, as shown in the photo below.
Now, try to separate your thumbs (representing the parents) – they will open, because your parents are not destined to live with you lifelong, and have to leave you sooner or later.
Join your thumbs as before and try to separate your index fingers (representing your siblings) – they will also open, because your brothers and sisters will have their own families and will have to lead their own separate lives.
Join the index fingers and try to separate your little fingers (representing your children) – they will open too, because the children will also get married and settle down on their own some day.
Finally, join your little finger and try to separate your ring fingers (representing your spouse). You may be surprised to see that you just cannot – because husband and wife have to remain together all their lives, through thick and thin.