This paper proposes a formalism for on-demand service selection and composition. It is based on the agent-oriented requirements modeling framework i*, which can be used as a means of studying the requirements and architectural setting for service-oriented environment. We argue that a social ontology such as i*, extended with a formal reasoning mechanism, offers better understanding to the social/organizational relationship in a component-based, on-demand service world. By representing explicitly the underlying assumptions, and essential factors of services, an informal requirements model in i* can automatically evolve and compose a new service on-demand with quality. Eventually, it will assist participants of an open service oriented platform such as SOA (Service Oriented Architecture) to make rationale communication, selection, and binding decisions.