Peer review is integral to the evaluation of grant proposals. Reviewer perceptions and characteristics have received limited study, especially their associations with reviewers' evaluations. This mixed methods study analyzed the unstructured comments of 270 experienced peer reviewers after they scored proposals based on mock overall evaluations written by the primary reviewer. Comments were coded for topical content and emotional valence blind to participants' characteristics. The most frequent comments were about their experiences with peer review with negative valence. Additional themes were identified within the content codes, including concerns about favoritism and inappropriate behavior observed in other reviewers. Reviewers who made negative comments gave poorer scores than reviewers who did not. Reviewer mindsets are understudied: negative moods and cognitions may affect reviewers' overall evaluative severity. Future studies should further investigate these associations.