Q: Why is a PhD essential to become a peer-reviewer?

Peer review requires an in-depth understanding of not only the subject and associated literature, but also a level of maturity that comes with experience. The reason for this is that the peer reviewer is going to not only evaluate a researcher's work, but also determine the fate of their hard work through their verdict. So not only what you write, but how you write it really does matter. 

Sometimes having experienced the submission-rejection-acceptance cycle yourself as an author a few times is what is needed to prepare you to be a peer reviewer. Sometimes it is solid mentorship - someone who will show you how they review articles and even add you as a co-reviewer. Usually, such opportunities do not come by until one has completed their Ph.D. or M.D. Indeed, many journals do not even consider peer reviewers who have not obtained a doctorate. However, this is very journal-specific, and even discipline-specific. 

If you truly wish to establish yourself as a peer reviewer, try the following: 

1. Write papers. Writing and getting published leads to more opportunities, including invitations as a peer reviewer!  

2. Request a professor to allow you to help with a peer review. Since peer review is confidential, it may be necessary for him/her to write to the journal and get permission for you to join as a co-reviewer. Yes, some journals do allow that. 

3. Try writing to the publisher as well as journal editors to request being added to a "peer reviewer pool." In your request, include your expertise and areas which you are comfortable in as a reviewer. 

4. Step out of your comfort zone - try writing to a variety of journals, not just the top ones in your field. After all, you want to first get your foot in the door, don't you?