This research explores the link between achievement emotions and covariational reasoning, a type of mathematical reasoning involving two variables. The study employs a case study approach, focusing on a high school calculus student named Valeria, and develops a theoretical framework based on the control-value theory and levels of covariational reasoning. The results reveal that Valeria's ability to coordinate variables and her perceived importance of solving mathematical problems influence her experience of achievement emotions, including enjoyment and frustration. The case study enables the creation of a hypothetical model that explains how students feel achievement emotions when tackling mathematical tasks that require covariational reasoning. The study highlights the significance of comprehending the interplay between emotions and mathematical reasoning to cultivate advanced cognitive and emotional abilities. This research is essential in bridging the gap between emotions and mathematical reasoning, a topic that has been previously overlooked in research on the affective domain.