The thing about software testers is no one was formally taught to be one in school; those are good tend to fall into it.
If you are ever lucky enough to work with one, you should have a very, very healthy fear of professional testers. They are terrifying. Just scan this "Did I remember to test" list and you'll be having the worst kind of flashbacks in no time.
Developers are severely uneducated about working with testers.
I believe a key turning point in every professional programmer's working life is when you realize you are your own worst enemy, and the only way to mitigate that threat is to embrace it. Act like your own worst enemy. Break your UI. Break your code. Do terrible things to your software.