Many times in a marriage one spouse may be unaware of any problems between them. There can be little things that irritate and disagreements that come up but you can still be oblivious to what is going on.
Most of the time a marriage will fail over a period of time as opposed to one sudden event. It erodes from all the little things that just build up without either party trying to mend things. With out tending to each others emotional needs you can tend to drift apart.
Although communication or the lack of it can be a problem, many times the lack of conflict resolution can take a toll. Eventually this can lead to complete conflict avoidance because one or both of the couple just gives up in despair.
Over a period of time the couple can become unwilling to discuss with each other their feelings and intimacy will break down. One of the persons may regard every issue that comes up as a means to bully the other till they give up.
When a couple no longer has sex and there is no reasonable reason for it, there is most likely an emotional disengagement between them. Or possibly a sign there is infidelity in the marriage.
When a marriage is disintegrating, a person may turn to things like work or their children to fill the void in their life. The children can become the center of their life or they may become workaholics which can lead to other problems.
With the thought that single life is ahead, many people will start an exercise and diet program, buy new clothes or change their hairstyle. Some may start a new activity on their own to start a new social network for them.