The first reason is that we know too much about each other. We intimately know the likes, dislikes, thinking patterns, priorities, preferences, values, goals of each other. Their body language communicates the whole heap of stuff to us. Hence, there is seldom a need for detailed discussions and debates.
The second reason is that the other person does not listen. So our feedback, views, and suggestions do not matters. Our words fall on deaf ears. So, we have given up on them. As a result, we say yes to most things they say. We keep our opinions to ourselves.
To the outsider, it looks like it is a harmonious relationship. In reality, the relationship is superficial because one has given up on it.
What has been your experience?