"Husband and wife – if they merit, God's presence is found between them." (Sota 17a)
I can recall my parents occasionally having arguments, sometimes bitter, with each other when I was growing up. But personally, I almost never get into arguments with anyone. How is this? Am I really a better person than they are?
The more I learn about life, the clearer it is that the answer is not a quick and satisfying “yes”. The reason I do not argue is simple, and reflects my circumstances more than my character.
It is often hard to know what a single person is thinking. People have many reasons to keep their thoughts private. But in a marriage, much more so than any other relationship between individuals, thoughts do not remain private. They are shared verbally with one's partner.
The arguments which I occasionally witnessed in my family, and which are probably present in every family, are the result of frustration and anger. I certainly do experience these emotions sometimes. But I generally keep them to myself. If I were married, things would be different. I would much more readily express negative emotions to my wife than to other people. She might respond negatively, again more readily, and I might reciprocate. And just like that, we would get into a fight. The negative character traits which caused this fight were always part of each of us. But only when we were together would they express themselves in words and actions, immediately visible to anyone around us. Right now, I do easily not get into arguments and fights. But once I'm married (or in a sufficiently serious relationship), I can expect to do so more often.
What is true for negative character traits, like selfishness and cruelty, is also true for positive ones. With one's spouse, these positive traits too will be expressed more openly, and visibly to people other than the married couple.
In my opinion, this is the basis of the gemara saying that God's presence dwells between the husband and wife.
Apparently there is some set of Godly actions and attitudes – love, generosity, kindness, and so on – that wherever they are present, we say that the Divine presence is there as well. Each person, living by themselves, may possess these character traits, and thus we call them a holy or Godly person. But this Divine presence is private, as the character traits generally are. When husband and wife with these character traits are together, the traits are most strongly and visibly expressed, and the holiness become shared and evident between them.
There is a famous Romantic poem entitled “She walks in beauty”. It is not obvious whether a woman's beauty is part of her, or part of the mind of the man gazing at her. Either way, surely the beauty is present whenever he sees her. Therefore, he may speak of her walking “in” the beauty, it being part of the fabric of the situation in which they meet. We may speak the same way of God's presence dwelling between a married couple. When they interact with each other, treating each other with kindness and respect, the situation is one of Divine attributes and Divine presence. We cannot say that God is physically located anywhere, but when husband and wife treat each other this way, Godliness is surely an element of their interaction.