The Men's INTernet Study II included a randomized controlled trial to develop and test an Internet-based HIV prevention intervention for U.S men who use the Internet to seek sex with men. In 2008, participants (n = 560) were randomized to an online, interactive, sexual risk-reduction intervention or to a wait list null control. After 3 months, participants in both conditions reported varying degrees of change in sexual beliefs or behaviors. Using content analysis and logistic regression, this mixed-methods study sought to understand why these changes occurred. Level of critical self-reflection of assumptions appeared to facilitate the labeling of sexual beliefs and behaviors as risky, which in turn encouraged men to commit to and enact change. New HIV prevention interventions should include activities in their curriculum that foster critical self-reflection on assumptions.