The person who hears every complaint about a relationship may never hear what makes it worth keeping.

The person you vent to hears only the difficult half. They receive every grievance and none of the ordinary good, and over time they build a picture of your relationship that you would not recognise. Then they respond to that picture, and their advice starts to seem oddly harsh.

So tell them the other half too, occasionally and deliberately. It is not disloyal to your own complaints; it gives your confidant a true model to work from. And when a friend tells you only bad things about someone, be careful about forming a verdict, since you are working from a sample that was never meant to be representative.