Upper secondary students' task solving reasoning was analysed with a focus on arguments for strategy choices and conclusions. Passages in their arguments for reasoning that indicated the students' beliefs were identified and, by using a thematic analysis, categorized. The results stress three themes of beliefs used as arguments for central decisions: safety, expectations and motivation. Arguments such as "I don't trust my own reasoning", "mathematical tasks should be solved in a specific way" and "using this specific algorithm is the only way for me to solve this problem" exemplify these three themes. These themes of beliefs seem to interplay with each other, for instance in students' strategy choices when solving mathematical tasks.