This paper discusses rational interaction as a methodology for designing and implementing dialogue management in ambient environments. It is assumed that natural (multimodal) language communication is the most intuitive way of interaction, and most suitable when the interlocutors are involved in open-ended activities that concern negotiations and planning. The paper discusses aspects that support this hypothesis by focussing especially on how interlocutors build shared context through natural language, and create social bonds through affective communication. Following the design guidelines for interactive artefacts, it is proposed that natural language provides human-computer systems with an interface which is affordable: it readily suggests the appropriate ways to use the interface.