The 2 main reasons are lack of communication and third party intrusion.
Communication between 2 people in a relationship is key to the success of that religious. Constant communication will enable you know more about the other person. Relationships with little or no constant communication end up folding up because most times, you see them having problems that would have been solved if you interact often. A fast way to fully understand another person is by interaction.
Third party intrusion
Many relationships have collapsed due to either parties allowing third party opinion. In any successful relationship, there will always be other people that wish to be in that shoes. They concoct lies or deep dig to find some secrets about the other person in a bid to destroy the relationship. You as a person, you should talk with your other half regarding the issue and what caused it and so on. Some people won't do that. They will just take the story from the third party and confront their other half angrily. This have a negative effect on the relationship. Don't give room for third parties in your relationship.