Software testing is crucial for ensuring software quality, yet developers' engagement with it varies widely. Identifying the technical, organizational and social factors that lead to differences in engagement is required to remove barriers and utilize enablers for testing. While much research emphasizes the usefulness of software testing approaches and technical solutions, less is known about why developers do (not) test. This study investigates the first-hand experience of developers with software testing. The study illuminates how developers' opinions about testing and their testing behavior changes. Through analysis of personal evolutions of practice, we explore when and why testing is used. Employing socio-technical grounded theory (STGT), we construct a theory by systematically analyzing data from 19 in-depth, semi-structured interviews with software developers. Allowing interviewees to reflect on how and why they approach software testing, we explore perspectives that are rooted in their contextual experiences. We develop eleven categories of circumstances that act as conditions for the application and adaptation of testing practices and introduce three concepts that we then use to present a theory of emerging testing strategies (ETS) that explains why developers do (not) use testing practices. This study reveals a new perspective on the connection between testing artifacts and collective reflection of practitioners, and it embraces. It has direct implications for practice %and contributes to the groundwork of socio-technical research which embraces testing as an experience in which human- and social aspects are entangled with organizational and technical circumstances.