University students are frequently required to collaborate, often in the form of collaborative writing tasks. The process as well as the outcomes of the collaboration depend on choices made during the group formation phase. Studies on why students select partners for collaborative writing tasks are, however, lacking. Therefore, the present study aims to gain insights into (1) university students’ preferences with regard to teacher-assigned and self-selected group formation, (2) which motives they take into account when self-selecting a partner, and (3) the degree to which students select a partner similar to themselves. Sixteen dyads collaboratively wrote a research paper. Prior to the collaboration, 30 students individually completed questionnaires and 28 students were individually interviewed. The findings show that most students have mixed opinions regarding teacher-assigned or self-selected group formation (n = 18), while the others bar one prefer to self-select a partner (n = 9). Students’ main motive for self-selecting is familiarity, and, more in particular, prior collaboration experience with a specific partner. Other motives include friendship, ability, convenience, and attitude. Furthermore, students tend to select a partner with a similar attitude, ability, task approach, and perspective towards the content of the task. Predictability seems to be the most important driver for self-selection.