Despite extensive evidence reporting the numerous health benefits of physical activity, recent studies show that many people (60%) do not follow the recommendations to meet the accepted minimums of physical activity. Some of the main problems in today’s society are the high dropout rates (35%) and low adherence to the conditions for achieving the physical activity recommendations. The aim of the study is to analyze the reasons for dropout from sports that the participants particularly liked, to design a new scale of reasons as to why they dropped out based on several previously existing scales, and to study the dropout differences based on different variables. The sample consisted of 813 Vizcaya university students (61.6% women and 38.4% men), aged between 18 and 29 (M = 20.5; SD = 1.8). Frequency distributions, univariate descriptive analyses, and bivariate statistical analyses, such as t-test, analysis of variance, and correlations, were analyzed. The construct validity of the dropout reasons questionnaire was analyzed by combining an Exploratory Factor Analysis (n = 406) and a Confirmatory Factor Analysis (n = 407). The five-factor measurement model was appropriate and allowed to identify the factor rankings and its relation to some key variables. Lack of time (or dedication to other activities unrelated to physical activity) and fatigue seem to be the main factors for dropping out. The life changes produced at the time of entry and during university life seem to guide students to value other activities.