First, all relationships change over time. Falling in love is like tinder; staying in love is kindling and fuelwood. The first never lasts long, but you can't light the others without it. Unfortunately most people perceive that as a decline and bail out, shifting from one shallow relationship to another. Building something deep and meaningful takes not only time, but skill. You have to know about those transitions and work with them. Merely trying to hold onto exactly what you had in the beginning does not work.
Second, successful relationships require growth. People get more interesting over time as they discover new things. A couple (or moresome) should explore things together and find more common ground. Cultivate different interests, too, because nobody can be everything to someone else.
Third, don't keep re-having the same fight over and over again. Strive to find some little thing you can change. That way, decades down the line, you will still be finding new and exciting mistakes to make.
If you're not improving, not helping each other grow, then why are you even together? Those shallow relationships don't last.