I’d suggest you stay focused on the mutual purpose of improving your work relationship. The person in the original post who was getting worked up clearly felt their status or self image was being threatened, which meant they couldn’t hear the feedback. Same will happen if you try to convince your coworker that it’s ‘normal’ for you to be dissatisfied with their work. Focusing on the fact that you respect them and want to maintain a good working relationship will help them believe you have positive intent and a mutual purpose, and they are more willing to then want to understand how they can continue to build on that. Not sure what aspect of their work you are dissatisfied with, so that may be hard to broach, but suggesting an alternative for how they could do something rather than criticizing how they are doing it now would be a good general rule.