One thing that causes separation is that the flaws of a person do not come out until after they have been together for a while,and some people hide these flaws,but after a while they come gushing forth as if bursting out of a dam for it is part of their personality.
If they are kind of mean,and hateful,and kind of think the opposite sex is kind of not trusting,or whatever fault with them,but do not want to be alone,they will hide their faults temporarily,and then they come out after they been together for a while.
Which you hear some say they changed,or that is not the person I married.
People hide their flaws,and are on their best behavior when they first get together,for if they did not hide their flaws,and was not on their best behavior,then nobody would get together,and then the flaws,and crappy behavior come out later after they hook the person,and been together for a while,for they cannot hold the flaws back because it is their personality.
If they are mean,hateful,towards people,and bitter about the opposite sex,it will be hidden,but it will come out later as they display those flaws for it is part of their attitude.
Which is why so many people in marriage find that they cannot get along after a while,and were not really compatible in today's world compared to years ago,for people are less loving towards people than years ago when they got along better.
There is more distrust,selfishness,arrogance,lack of care for other people,heaping of money,and material things,immorality,today,than any other time in history,so marriages will not last as long.
Jesus said since iniquity shall abound the love of many shall wax cold.