Marriage takes work, and a lot of people aren't willing to work. It's not like dating, it's not like playing "house". It takes constant attention, compromise and communication to have a good marriage. I think a lot of people rush into marriage, some get married too young, and A LOT don't think marriage is for the long term. If you are getting married and thinking in the back of your mind, "Well, we can always get divorced later..." you shouldn't be doing it!
My husband and I sort of beat the odds, you could say. We only dated for about a year before getting engaged and we married young (21 and 22). But here we are, five years later happy as can be!