What is a key reason why developers have difficulty testing their own work?

One of the primary challenges developers face in testing their own work stems from the lack of objectivity. When developers build a feature or fix a bug, they naturally develop a strong familiarity with the code, which can lead to an emotional attachment. This attachment may cloud their judgment when assessing whether the code works as intended or if it meets the requirements fully.

Their close involvement with the code can result in a bias where they may overlook errors or be overly confident in its correctness. An external tester, who has no prior involvement with the project, can approach the testing with fresh eyes and a more critical perspective, reducing the chances of missing defects that the developer might unconsciously dismiss.

This inherent lack of objectivity highlights the importance of having independent testing roles in software development, as these roles can provide a balanced assessment of the software's quality.