The decisions of company managers are often not guided by the regulations governing the environment but by subjective factors that exert their effects in a hidden way in the decision-making manager's behavior. Market behavior is objectively controlled by regulators, but subjective factors lead to poor decision-making in some cases. On the one hand, the principal-agent problem is revealed behind the subjective factors, and on the other, cognitive biases affect the decision-making leader as a person. Cooperation with competitors from the same sector and sustainability issues are two similar areas where there is an opportunity to examine these concerns and where the nature of companies' decision-making shows similar or identical patterns on the basis of which decision-making behavior can be explored, characterized, and understood. This thesis examines the attitude of the managers of two existing Courier, Express, and Parcel market players regarding possible market cooperation. In-depth interviews and the completion of a unique questionnaire that revealed the presence of cognitive bias were the basis for the research. During the investigation, the cognitive bias influencing the decision of the two company managers was identified, and it was also examined whether the parent company or the owner's opinion affected their decision. The two case studies demonstrate how managers' decisions to develop collaborations are influenced by their prior experiences and prejudices.