Default thinking is the natural, preferred approach for a tester when they look at a problem. It is the familiar path the mind takes based on its knowledge and experience. Everyone has their own starting point that feels comfortable. Whatever it is, it is just the first step of thinking.
Default thinking is not wrong. It is a habit, shaped by experience, that develops over time. The important part is recognising that this first step is only the beginning of thinking. Testing becomes deeper when you think from different perspectives. Once you pass your starting point, you can choose to explore in other ways. There are many mindsets to learn, and you can strengthen your thinking with practice.
The aim is not to remove the natural tendency. It is to expand it. When you understand your default, you can use it as a starting point rather than a place to stay. This gives you the freedom to choose the mindsets that best fit the problem.