Despite the abundance of research on negative intrusive thoughts, far less is known about their positive counterparts. In two studies, we explored how positive spontaneous thoughts are related to people's favorite activities (i.e., their passions, Vallerand et al. in J Pers Soc Psychol 85(4):756-767. doi:10.1037/0022-3514.85.4.756, 2003), and how passions and associated thoughts contribute to mental health. In Study 1, 170 participants reported on the valence and frequency of the spontaneous thoughts they tend to have about several activities. Harmonious and obsessive passion both predicted more frequent spontaneous thoughts, but only harmonious passion predicted thought positivity. In Study 2, 232 participants completed measures pertaining to physical activity, spontaneous thoughts, and mental health. As predicted, positivity of spontaneous thoughts about physical activity predicted frequency of physical activity. Further, positivity of spontaneous thoughts about physical activity mediated the relationship between harmonious passion and indices of mental well-being.