I disagree as well.
In my experience, the dynamics of a relationship can change from minute to minute, as well as day to day, and year to year.
My love for my wife is constant and steady, and rarely alters, wheras she can hate me severely one day and love me loads the next, depending on a variety of circumstances.
It comes down to the differencce in out personalities - we are all a product of our upbringing and temprement.
I certainly don't believe in an inevitable 'fizzling out' - I think you reach a stage where you know you are going to be together for the rest of your lives, and you accept the pluse and minuses that go with that situation.