High prevalence of teenage pregnancy in low-income countries impacts health, social, economic, and educational situations of teenage girls. To acquire better understanding of factors leading to high prevalence of teenage pregnancy in rural Lindi region, Tanzania, we explored perspectives of girls and key informants by conducting a facility-based explorative qualitative study according to the grounded theory approach. Participants were recruited from Mnero Diocesan Hospital using snowball sampling, between June and September 2018. Eleven pregnant teenagers, two girls without a teenage pregnancy, and eight other key informants were included. In-depth interviews (including photovoice) and field observations were conducted. Analysis of participant perspectives revealed five main themes: 1) lack of individual agency (peer pressure, limited decision-making power, and sexual coercion); 2) desire to earn money and get out of poverty; 3) dropping out of school contributing to becoming pregnant; 4) absence of financial, material, psychological, or emotional support from the environment; and 5) limited access to contraception. A majority of girls reported the pregnancy to be unplanned, whereas some girls purposely planned it. Our findings and the resulting conceptual framework contribute to a new social theory and may inform national and international policies to consider the needs and perspectives of teenagers in delaying pregnancy and promoting sexual and reproductive health in Tanzania and beyond.