Have you ever compared yourself to someone else? Maybe from a magazine, maybe a friend? In a relationship you can do the same thing. You can find yourself looking to someone's relationship, hoping to have something similar.
The key to being happy in a relationship is to know what your relationship is made of, and to be happy with what you have. Whether you are single, married, or somewhere in between, be happy with each stage of life.
I have been blessed with multiple couples to help give me advice. I have never felt like I needed to make my marriage like someone else's though. I am happy with Andrew, and he is happy with me. You need to be as happy with where you are! Accept that no one has a relationship that is perfect, and that you should embrace the imperfections of your relationship and yourself. That allows you to have the best relationship you can have.