Most Helpful Guy
Lack of trust for each other. Lack of conpromise. Lack of clear communication or lack of communication. Unwilling to cooperate and work together on the relationship, mistakes and fighting can and will happen but both parties must be willing to forgive each other and keep moving forward and not constantly holding grudges towards each other and constantly blaming each other. Having unrealistic expectations and goals for the relationship and as well as for themselves and their partner.
Also, the younger someone marries the less likely the marriage would last long the last I've checked. And then sometimes it takes a second time or third or more before they finally found each other to be a perfect fit for each other. Other times they don't and it never fit or ever works out no matter what.
Most Helpful Girl
Selfishness is one of the key factor in a deteriorating relationships. Many people are just in it for all the wrong reasons. Especially when it comes to a sexual relationship. It just becomes harder to move on.
Failed relationships are Intentional. Somebody has to want to get out somehow in someway. They don't usually know how or when, but they just do. Cheating is intentional, as well as a person deliberately pulling back.
Bitterness and Un-forgiveness is another one. As many times I have spoken to people for years, there is always a person or both that just can't let go from another persons transgressions. They don't forgive and their angry, secretly bitter. It's a poison just waiting to be released. It's enough to stagnate the relationship and making it unhealthy. Which brings me to another point.
Stagnation of the Overall growth in a relationship is another problem.