In the world of fairy tales, the Prince or Princess find each other and after overcoming small obstacles live happily ever after.
Many go into marriage with an idealized version of what it should be and end up bitterly disappointed when their "happily ever after" is not what they expected.
The first thing to remember is marriage takes work. It takes patience, sacrifice, and forgiveness. The flood of feel-good romantic feelings eventually simmers down. What is left is the real world marriage of two people who while they may have many things in common also have many things that differentiate them.
In a longstanding marriage, the key is to set one's mind to keep the marriage alive. In the day to day world, the top priority should be to continue to work on a united front.
Many people grow apart when life gets tough. They lose themselves in work, or their children, and forget to stoke the embers of their marriage. Once their children leave, or they retire, they find that they are married to a complete stranger.
It is all too easy in the American mentality of instant gratification to divorce or end a marriage if it doesn't live up to certain expectations. A true marriage that will last is not going to be based solely on passion but on an intense commitment to each other regardless of life's circumstances.