Suffering and unhappiness are caused by attachment, according to Buddhist philosophy; attachment to impermanent things. In the case of marriage we say “until death do us part”, which recognizes the impermanence of life. But in the case of separation and divorce we can suffer because it happens during the period where we expected permanence – the period “until”.
It is the nature of the left hemisphere of our brain to analyze the past and make plans for the future based on assumptions of some degree of continuity – of the relative continuance of situations and things and people. If we want to compare our brains to the way computers work, the left hemisphere runs on a serial bus whereas the right side is more like a parallel processor. The left side tells us to put money into a 401(k) and save for retirement and defer taxes. It tells us that buying a house and having deductible mortgage interest will benefit us in the long run compared to renting. And it tells us that getting married will add stability to our relationships and our lives.
How to avoid suffering and unhappiness involves not being attached to those things which are not permanent. The only reality exists in the present moment which doesn’t even have a fraction of a second of time associated with it.
The answer isn’t to be oblivious to the future and avoid all planning of course. We are human beings, and as such have to plant seeds if we want to have a chance at harvesting.
But if we live entirely within our left brains and buy into the illusion of permanence, there will be times when we are unhappy and suffer as a result. We will be mourning the loss of something we never in reality had.