From the Nerdery’s documentation on code review guidance:
Be kind. Yes, a person is not their code, but writing code is a creative act. Creators are by nature intimately involved with their creation. You’re going to be critiquing another person’s creation, at a minimum, be kind when you do it. Better yet, find positive as well as negative things to say. We want both the code and the coder to be better once you’re done; don’t make it easy to reject your constructive observations by being a jerk about it.
Lot’s of other great advice in the linked document, but this one really resonated with me.
Eric L. Barnes 