When a relationship is going good, what each individual asks of the other is usually fulfilled, often with enthusiasm, if realistic.
When a relationship goes bad, each request or claim becomes a jarring insult.
Why do relationships go bad? Well more than likely one or both participants perceive (whether accurately or not) an imbalance in shared power. One has slighted the other in some way which is not being repaid. There are plenty of other reasons really. Something else to investigate.
