This article examines why Asia-Pacific middle powers cooperate with each other on security issues. The article challenges the assumption that middle powers are primarily influenced by great-power structural factors in their security relations with each other. It argues that the dominant structural explanations of security cooperation between secondary states—balancing against the rise of China, responding to burden-sharing pressure from the United States, or hedging during a period of great power transition—have not been sufficiently tested. Using the 2009 Australia-South Korea security cooperation agreement as a representative example of middle power security cooperation, it finds that inter-personal leadership preferences played a key intervening role in filtering existing structural incentives into actual policy cooperation. It shows how congruence in leadership preferences shapes when and with whom middle powers are likely to cooperate on security issues. The article opens up the research agenda on middle powers by examining how they approach security relations in bilateral and horizontal contexts with each other.