How does a romantic relationship gradually turn into a platonic one?

A colleague said that after being married for a long time, they finish and leave, sleeping in separate rooms.

Mainly because his wife needs to sleep at 10 PM and wakes up at 6 AM. Sleeping with her means no lights, no noise, no big movements, otherwise she'll nag and it's noisy.

She won't turn on the AC even in 30-degree summer heat. If you turn it on, she nags. When you leave, she secretly turns it on in the middle of the night.

Sleeping with her feels like being in hell. Even if you have a cold and cough at night, she'll nag. Every time she nags, it's about how wronged she feels, how she has to work tomorrow, and how you're keeping her awake.

Their room has an ensuite bathroom. She can go to the bathroom several times herself in the middle of the night. But if my colleague goes to the bathroom at night, the sound of flushing after he's done will disturb her and she starts nagging again.

The colleague said he needs sleeping pills to sleep with his wife. Only after sleeping separately did he feel liberated.