Here's my two cents worth. I can speak from experience in this matter. My partner and I celebrated our 30th anniversary in February. I was and still am amazed at the number of people who crave a LTR, it's the thing of their dreams, yet they have no idea how much work it takes to make one go. You basically have to have the ability to turn the other cheek when necessary to keep peace and harmony in a household. You must be able to choose your arguments carefully. Somethings are just not worth arguing over.
In our house we have had numerous incidents that would have caused most other couples to call it quits. Infidelity is the one that sticks in my mind most. My partner announced on day that he met someone and had fallen in love and was leaving me. He did leave me, we separated for several months. During our separation we continued to talk about things, we realized that we were both at fault in one form or another for the affair, we decided we loved each other more than anything and decided to give it another go. That was five or so years ago. We are still together. I read and hear stories of couples separating for the most minor things that it makes me laugh to think there was a relationship to break up at all. Living a life together means just that, each person lives their lives, life throws curve balls and you have to want to handle them together. We both had and still have stressful successful careers, managing those, a home, friends, family, money and everything that comes along with it adds to the stress.
What is the moral? I think that there has to be a very strong commitment to one another, a very strong sense of love and devotion helps too.