Emotions are significant in the process of becoming a teacher, especially during the teaching practicum. While studies have repeatedly shown that pupils impact the emotional experiences of student teachers, little is known about student teacher emotions that are triggered by social interactions with their mentor teacher and their team partner. This is the focus of the present research. It is investigated: (1) which emotions are experienced in social interaction situations in the practicum, (2) which factors trigger these emotions, and (3) based on self-determination theory, how the evoked emotions are linked to the fulfilment of basic psychological needs. In order to explore these research questions, semi-structured qualitative interviews with 27 Swiss student teachers were conducted. Thematic qualitative text analysis shows that in different interaction situations, such as successful teaching-related cooperation, support, positive feedback, and goodwill of the mentor teacher, positive emotions are triggered, which are strongly connected to need fulfilment. On the contrary, situations of failed communication, negative feedback, and lack of support are related to need threat and evoke negative emotions. This study shows the importance of emotions in interaction situations during practica and the need to focus more strongly on emotional dimensions of becoming a teacher in teacher education.