Preferences by mathematics from secondary students is very low in some countries, this fact makes governments and teachers require information to make the best decision about this problem. In this study, we respond the question: Why do any secondary students prefer mathematics? By applying the center-point triangular whitenization weight functions (CTWF) method, which is based on grey systems theory. A case study was conducted on featured students who were finalists of a Mathematics National Competition (CONAMAT), which was held in Lima, Peru. Eight criteria were studied. The results revealed that for the consulted students the criteria that influenced in their preference by the mathematics were the motivation from students (C1) and the vocation for science or engineering from students (C8). In addition, the criterion the influence of mass media (C6) was less valued, as it does not contribute to motivation and studding of mathematics. The results from this work could help to governments, directors, and teachers to make the best decision on how to improve the motivation and preference of secondary students to mathematics.