Here are some sample expressions I found on-line
My spouse is not abusive, mean, a bad person, an adulterer or any of those other labels. My spouse refuses to grow up. Wants to be a child. Resents working all day, coming home, cooking dinner, cleaning up, dealing with children and the other such things that come along with being an adult
I’m a divorced man who didn’t want to be divorced. I wasn’t abusive, didn’t commit adultery, etc. Among other things, she said her life was just moving a different direction. I don’t remember her saying that she wasn’t happy, but I know she wasn’t. However, divorce only complicated her problems
The spouse says "I'm just miserable. I'm not happy. You don't make me happy!"
I think there are so many permutations and combinations...it depends on the reasons for being unhappy. If the marriage didn't start off on the right foot and continued along those lines, if one partner is a serial cheater or has addictions or emotional problems they refuse to work on etc. yes, those are definitely reasons for divorce.
I agree: unhappiness shouldn't cause people to rush into divorce. But sustained unhappiness, where you have clearly told your partner about your needs and your partner chooses to do nothing?