Why About a Half of Marriages Break Down?

When a couple standing on the red carpet to swear their marriage in front of the cheerful audience, no one at that moment is anticipating that the marriage will have about 50% chance to break.  Eventually, about a half of marriages in the US break.  People have identified many causes, such as issues related to sexual life, religion, economic condition, personal interests, personality, etc.  Although these are important causes, it remains unclear why not 5%, or 10% or 80% of the couples divorced and why about 50%?

If we rank all men and women by their tendency to be in marriage from least likely to most likely, this will form a bell-shaped distribution, just like the distribution of the height and weight of all people.  We now assume that people are free to choose their marriage, this will ended up with a half of the marriage with a mismatch: either a man with high tendency to stay in marriage married a women with low tendency, or vice versa; and about a half with a good match.  While couple couples with a good match will stay in marriage, couples with mismatch will end up breaking down.