Keep a disagreement about the issue in front of you
A disagreement becomes harder to resolve when one action turns into a verdict on someone's character. The kitchen was left untidy describes a situation. You are always selfish adds a much larger accusation. Keeping the issue specific makes it clearer what needs to change.
Go deeper
Describe the behaviour without making it the whole person
Two people disagree about a shared kitchen. One says that dishes were left overnight after an agreement to clear them. The other can understand and respond to that specific account.
Calling the person selfish or impossible changes the subject from a behaviour that can be discussed to an identity they may feel compelled to defend.
Specificity does not require minimising a repeated problem. You can say that it happened on three evenings and explain its effect without treating the behaviour as a complete description of the person.
The discussion can then address an actual arrangement: who clears the dishes, when it happens and what to do if the plan cannot be followed. Character accusations offer much less practical direction.