Why do over 90% of the learners in Massive Open Online Courses (MOOCs) never finish the course? There is a need for further studies focusing on the learners' experiences of participating in MOOCs and factors that influence the decision to complete or drop out of the course. To deepen our understanding of why learners complete or drop out of MOOCs, we report on a qualitative case study based on in-depth interviews with 34 learners with different degrees of course completion for two MOOCs. A qualitative analysis of the interviews led to the identification of four main factors influencing dropout: (1) the learner's perception of the course content, (2) the learner's perception of the course design, (3) the learner's social situation and characteristics, and (4) the learner's ability to find and manage time effectively. How the learners conceptualized a MOOC had a strong impact on how they engaged with the contents. We discuss the implications of our results for MOOC practice in terms of time, openness and accessibility and provide recommendations for future research.