There could be many things making testers lack of confidence about the product's quality. But in many people’s eyes, the only way to tangibly prove the product has quality issue is to find new bugs. However, once those bugs are fixed, people start to believe the product quality is now good. What if testers are still not comfortable with the quality? Eventually, to prove to those who asked “why do you think there is still quality issue if you cannot find any quality issue”, testers would have to find a few more new bugs. That would never end.