@rands had an interesting tweet:
“Doubt is the best QA.”
When you are doing quality-assurance testing, doubt is what will cause you to check (and recheck) all sorts of things. They may work just fine, but you aren’t sure, so you check. This is a great attribute for anyone who is testing the quality of a thing.
If you look at the corollary, it may become more clear:
“Confidence is the worst QA.”
Do you want your testers to come from a place of confidence (“Of course it works!”) or of doubt (“I’m not so sure it works.”)? I vote for the latter. They will be far more careful and complete in their investigation.