Testing is a separate skill and that’s why it can be frustrating
The best testers that I know really enjoy breaking stuff. They have this special mission to break anything they get their hands on. They take great pride in finding the stuff that the developer probably didn't think of. They smile when it behaves weird, they laugh when it crashes. Their ego rises when they tell the developer that they were smarter.And therefore, the one who built is, is not the person who makes it his lives mission to break it. Breaking it means more work, more searching for bugs, and then testing again. And nobody likes doing that. Except for awesome testers. They love looking at the face of a desperate developer getting frustrated by a nasty bug.