Good peer reviewing entails providing open and substantial criticism in a constructive environment. It's important to highlight strengths while framing suggestions for improvement in a positive manner, so the writer can maintain originality and hone their skills.
When peer reviewing, constructive criticism is key. Critiques should offer specific suggestions for improvement without being discouraging towards the author's work.
Peer reviewers should be people you trust whose opinions you value. Still, you should try to appreciate any opinions on your writing, while taking them with a grain of salt. Remember, there isn't really a right answer with writing, just a better way of saying things.