There are many reasons why couples who were once inseparable decided to part ways but whatever those reasons are, one thing is for sure, those reasons didn't come up just overnight. Many marital conflicts were ignored until they get so big that couples cannot handle them anymore until they explode and eventually decided to part ways.
You need to be able to understand your spouse. You have to know what their wants are and needs from the marriage. To do this you need to be able to go to areas where they could feel vulnerable and exposed. It might not be a particularly comfortable thing to do but the more that you can understand your spouse the more you can help them to grow, and the more you are able to give comfort and support. You are both in this, together, for the long term and to achieve a happy, loving and fulfilling marriage you have to be available for each other, if neither of you understands the other, what can you achieve?