The design of assistive environments often follows technological potentials and constraints in order to react on people's behavior and assist them in everyday life situations. In this paper we discuss the role of mental models for improving the usability and user experience with assistive environments. We argue for a user-centered design including interaction design which provides intentional support in building, shaping and confirming the mental model of the user. The paper aims to motivate a wide discussion on design principles for assistive environments focusing on better user experiences and assistance qualities.