Why do relationships deteriorate?

Because partners OBJECTIFY each other - they exploit one another to satisfy all their desires and expectations. When the honeymoon phase passes and people begin to realize that the reality does not match expectations, scandals take place. Many have daily ones over every little thing. This is how the self-preservation instinct works: when you are exploited and exploit someone, over time, the psyche starts to perceive this as a threat. Thus, people begin yelling, subconsciously provoking each other to break up.