What makes people’s interest in doing an activity increase or decrease? Regulatory fit theory (E. T. Higgins, 2000) provides a new perspective on this classic issue by emphasizing the relation between people’s activity orientation, such as thinking of an activity as fun, and the manner of activity engagement that the surrounding situation supports. These situational factors include whether a reward for good performance, expected (Study 1) or unexpected (Study 2), is experienced as enjoyable or as serious and whether the free-choice period that measures interest in the activity is experienced as enjoyable or as serious (Study 3). Studies 1–3 found that participants were more likely to do a fun activity again when these situational factors supported a manner of doing the activity that fit the fun orientation — a reward or free-choice period framed as enjoyable. This effect was not because interest in doing an activity again is simply greater in an enjoyable than a serious surrounding situation because it did not occur, and even reversed, when the activity orientation was important rather than fun, where now a serious manner of engagement provides the fit (Study 4a and 4b).