I don't forget the day that I saw John walking up the side aisle of our store, and I thought, "This is a separate person, with a separate brain, and I shouldn't be surprised if it doesn't think the way I do." Seems obvious once you think about it, but in practice, not so easy. In a bunch of years being married, and a lifetime of being around people who were, there are some things I noticed:
No matter how well you think you know your spouse, there are things you don't know: everybody has certain expectations of what marriage and families are like, and there are always differences. Assuming that you know what to expect of other people can bite you. For example, learning how different families are used to doing holidays, and planning vacations, and what is expected of vacations and how do you spend a Sunday afternoon, takes communication, explanations, and patience. One factor in a failure of a young marriage in my extended family was that the husband's idea of Sunday afternoon was sleeping in an easy chair. His mother commented that is what his dad always did, and I'd seen his grandfather asleep in a chair after Sunday dinner as well. Life is full of things that we think "everybody" does, and we're usually surprised. People really don't read minds well at all.
Nagging, criticizing, and putting down your spouse in public is a killer. It's an easy habit to get into, but is hurtful to be the target, and hard to watch. I saw bad cases of this in two different couples at two family gatherings, and in both cases, the next time I saw them, they had divorced.
Probably the most troublesome part of dealing with an imperfect human being (and I remember telling my kids to remember that that's all we have) is anger. Again, expectations are part of this. In some cultures, people are not taught that it is possible to deal well anger. If someone makes you angry, it's your fault; if you get angry with them, that's your fault as well. I finally realized that the root of most of my anger was self-pity; and realizing that took all the fun out of it. I hit a point when I realized that I was angry enough to get myself into deep trouble (and I don't remember what it was I was so angry about). I remember grabbing onto the rail of a crib and praying desperately to have it gone. It disappeared like water in a sink drain. I realized that I had the choice of giving up anger. I also learned that ending an argument by one party giving up just to shut the other one up does not accomplish anything useful. There are times when something doesn't need to be settled right at that moment; often if you agree to disagree and drop it, it usually becomes apparent that it's something that doesn't need to be an issue. I've known people who have had a difference over something for many years that was eventually settled when reality hit.
God made us all different; it's amazing what a variety of people and personalities there are--often married to each other. One of the most encouraging things John ever told me, not long after we met, was "I know this, and you know that, so look at how much we both know!"