Motivating users is an important task for virtual agents in behaviour chance support systems. In this study we present a system which generates motivational statements based on situation type, aimed at a virtual agent for Post-Traumatic Stress Disorder therapy. Using input from experts (n=13), we built a database containing what categories of motivation to use based on therapy progress and current user trust. A statistical analysis confirmed that we can significantly predict the category of statements to use. Using the database, we present a system that generates motivational statements. Because this system is based on expert data, it has the advantage of not needing large amounts of patient data. We envision that by basing the content directly on expert knowledge, the virtual agent can motivate users as a human expert would.