I've spent the last couple of years at the two bottom levels of that scale. It took me an embarrassingly long time to realize one of the root causes: my wife and I have fundamentally different personalities that are at odds with each other. My wife is an extrovert. She is energized by going out, visiting places, doing things, meeting people. I'm an introvert. I get energized by staying in, reading a book, watching a…
It's obvious once you understand the differences but until then it's pure misery regardless of how much you love the person.
You're often either unhappy or making them unhappy for no apparent reason.