Numerous theories are used to explain why people engage in health behaviors. However, these theories have some limitations, such as assuming that health is the main motivation for engaging in healthy behaviors, focusing mostly on healthy versus unhealthy behaviors, and not taking into account situational factors. To address these limitations, we developed a new concept called motivational flexibility, which is the idea that people can have multiple reasons for engaging in a behavior, and the main reason for engaging in the behavior can change across occurrences of the behavior. I hypothesized that motivational flexibility would be associated with greater behavior adherence, based on flexible goal adjustment research showing that the ability to disengage from failing goals and reengage with new goals is associated with success meeting goals (Rasmussen, Wrosch, Scheier, & Carver, 2006). However, a pilot study of motivational flexibility examining fruit and vegetable consumption found that endorsing more motives was associated with less consumption, especially when the most important reason for consumption varied day-to-day (Darlow & Lobel, 2011). That study suggested that motivational flexibility may be detrimental to some behaviors. In the present study, student exercisers (n = 198) and cigarette smokers (n = 116) completed daily assessments for 14 days in which they indicated whether they exercised or smoked, and listed their reasons for doing so. Frequent variations among the most important motive(s) for smoking or exercising each day (shifting) was associated with greater frequency of smoking and enjoyment of exercise, respectively. However, shifting was associated with less enjoyment of exercise when the number of reasons listed for exercising each day was great. Results suggest that shifting may be indicative of an ability to adapt to changing environmental demands. However, the combination of having many reasons for engaging in a behavior and not feeling strongly committed to any single reason may be detrimental to adherence. Findings are discussed in light of situational and individual factors that may modify the influence of motivational flexibility on various behaviors.