Positive interpretations can be self-fulfilling prophecies, just as negative interpretations can be. Suppose someone asks a question, and he could mean a stupid question, or an interesting one. If we answer the interesting one, it may lead him to be interested in that and see the issue in the proper way, even if he didn't already. And we will be saying something more interesting and therefore better. Assuming the person means the stupid question, or even asking if he does, shows we think he is or may be stupid, and encourages him to see himself that way.
Positive interpretations help make life safer. For example, a child in need of advice, and partially confused about a moral issue, will want to be able to ask his parent questions and make mistakes about that issue without his parent deciding he is wicked. Rather, the parent should stick to the positive interpretation that the child is learning, and is not bad, and will be fine, and wants to be good. And most of the things the child says that seem bad won't be. Some will be glossing over an issue while focussing on a different one. Some will be harmless confusion about an unrelated topic. Some the child will be right about. Some, while the content is bad, won't indicate any defect in the child himself who's just curious about a bad thing.
Another issue is that being wrong about positive interpretations is less costly than being wrong about negative interpretations. That is why criminals only go to jail if there is no reasonable doubt: if there is any reasonable positive interpretation of events in which the man is not guilty, the risk of making a tragic mistake is too high. Similarly, to treat someone too well is nothing to be ashamed of, and no great harm will come of it. But to treat someone, especially your friend or child, too badly is a mistake you will regret.