This eHow article might explain most of what you are asking. Just some people find out over the years that they really aren't compatible. Booze ruined my last marriage but there are many different things that cause a divorce.
Because each person in the relationship deserves happiness, and maybe they won't find happiness with the other person, so a divorce is necessary. I think Divorce should be a happy occasion. The alternative would be a life of suffering with someone you don't want.
Marriage is supposed to be for love, not a trap.