Some couples gradually lose their connection with each other—erosion. This tends to happen with more established couples when the stresses of life, e.g., work or kids, obstruct their loving connection. Other couples tend to explode with each other. It may be the small things in life that they fight about or it may be particular themes that they have difficulty resolving, e.g., finances, in-laws.
If a couple persistently fights, over a long period of time, the weariness of this can also create disconnection. Whether a couple disconnects or is perpetually in conflict, they will need to do things differently—changing the patterns that no longer work. Professional intervention could help.