Objectives Planning is an effective self-regulation strategy. However, little is known why some people take up planning and some do not. Such understanding would help interventions to promote planning. We investigated how adolescents explain their (non) use of planning for physical activity after an intervention. Methods Qualitative content analysis was employed to investigate follow-up interviews (a purposeful sampling; n = 19 low-to-moderately active, vocational school students) of Let's Move It trial participants twice post-intervention: 6-8 weeks and 14 months post-baseline. In the intervention, planning was one of the key techniques used to promote PA. Results We identified seven categories linked to reasons for (not) using planning. Most were related to feelings anticipated to result from planning. Action- and identity-related concerns were also raised. The reasons for planning were that the plan (1) helps to clarify what to do and to get things done, (2) strengthens the feeling of autonomy, (3) promotes a sense of progress, ability and control over one's PA. The reasons for not planning were that (having) a plan may (1) feel forced and like an unpleasant duty, (2) take away life's spontaneity and freedom, (3) result in anticipated annoyance and bad mood if one fails to enact the plan, or (4) be an effective strategy for others but not for the interviewee. Conclusions Planning may not only link to behavioural control but also the sense of autonomy, and thus subsequent motivation. We suggest various strategies to promote planning, including challenging non-planner identity and harnessing social dimension of planning.