This study investigated the impact of student-selected pairs on emotional engagement and language-learning opportunities generated during collaborative tasks. Sixteen learners of English at a Hong Kong secondary school performed collaborative tasks in two pairing arrangements: student-selected pairing and teacher-assigned pairing (mixed proficiency). Data sources include transcripts of task interactions, a self-selection questionnaire that assessed learners' criteria for selecting a partner, and a post-task emotional engagement questionnaire. The results revealed that learners' choice of partner was primarily based on friendship, and that, compared to the teacher-assigned pairing arrangement, self-selection led to elevated levels of enjoyment. However, neither arrangement had an overall advantage in terms of generating language-related discussions. A qualitative analysis of task interactions revealed that student-selected pairs more frequently used the L1 to elaborate and resolve English language issues, but the friendship nature of these collaborations also led to more frequent off-task discussions, limiting their language learning opportunities.