When one spouse is satisfied (although not really happy), and the other spouse is unhappy, complaints tend to be seen as betrayal.

"I thought things were going just fine! How could you spring this on me?"

So then the unhappy spouse is suddenly responsible for upsetting the apple cart. They are the troublemaker, the malcontent. That's why, when things are not really bad (no yelling, fighting, affairs, wild spending, addiction, etc), there is a natural reluctance of one spouse to bring up the one thing that is making them unhappy. Of course, it is a powder keg, and eventually some spark will set it off. Withheld affection and sexual relations is the prime example.