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

Developers often face challenges when testing their own work primarily due to a lack of objectivity. When individuals create something personally, they may struggle to view it critically. This bias can lead to overlooking defects or issues that would be more apparent to someone who is not emotionally invested in the work.

Objectivity is essential in testing because it involves assessing the product without the influence of personal attachment or pride in one's own coding efforts. A tester or another developer who hasn't been involved in the creation of the code can approach the work with a fresh perspective and a critical mindset, which is crucial for identifying flaws.

The other options are related to various challenges in software development and testing but do not capture the fundamental issue of personal bias as effectively as the lack of objectivity does. For instance, while lack of technical documentation or training may affect a developer’s ability to test effectively, it is the inherent familiarity and emotional connection to their own code that primarily hinders their ability to critically evaluate it.