A collaborative system of systems (SoS) is formed when independent organizations decide to cooperate to achieve mutual benefits, while retaining independence of their respective systems. Each constituent system (CS) of the SoS has a set of capabilities, some of which they agree to potentially use in active collaboration with others. Such an active collaboration is called a constellation and can be seen as an instantiation of the SoS which is created to provide a joint capability. Constellations are thus the working-horses of the SoS, but due to the operational independence of the CS, they have a choice whether to join a certain constellation or not. This paper discusses the reasoning and world model that is necessary for a CS to make well-informed decisions to join and leave constellations. We argue that it is necessary for the CS to understand not only the surrounding environment, but also to have models of other CS’ world models as well as of their probable future actions. It must be possible to predict whether other participants will uphold their parts of the collaboration or may defect from it to join another more rewarding constellation despite the agreements made when joining the SoS. The reasoning in the paper is illustrated using examples from two different collaborative SoS in the transportation domain.