When a couple gets a divorce, there is often one person who was not expecting it to end that way. They believed the issues between them could be worked out, and they may turn inward to try and find answers. Some of them will exhibit anger, but others will find they are mourning the relationship they used to have. Their partner might have already found someone new and remarried, but they are busy crying over what was instead of moving forward.
The loss of a relationship affects each person differently, but it can be a deep wound for someone who truly loved their partner. No matter how bad things between them were, they might have always felt that just a bit of work on their differences would cure the problems. These are the type of people who are often optimistic in the worst of circumstances, so the shock of being single again can be devastating. Rather than finding ways to move on, they will continue to spend years dissecting what went wrong.
There are many steps people go through when they are mourning the passing of a loved one, and some of them are experienced by a person who was not really ready for their relationship to end. They might feel completely shocked and numb when their partner tells them it is over, and they are often unable to respond in a timely fashion. Some partners announce their intentions in this manner so they can have complete control over the divorce, but it does not always last as long as they wish.
While depression is often a part of the grieving process, it is also a large part of an unexpected divorce. The person initiating the breakup might count on their partner being depressed and unable to cope, but one false move on their part will move the person out of shock or depression and into anger that can cause them to lash out physically or with legal counsel.