I think a key to instilling positive code review culture is to make sure that the comments are based on some objective, such as readability and clarity, and not based on personal preference. In particular, if one person is reviewing code of another, it is not helpful for the reviewer to remark that they would have written it differently, if they had written it at all. Some people for some reason can't keep themselves from doing this. They should remember that they did not, in fact, write the change, so their personal style is irrelevant. If code is clear, obviously correct, well-tested, and correctly formatted, it LGTM.