[rmk]

You hit the nail on the head. It takes time and effort to cultivate the QA mindset, and to shift into it from a developer mindset. Doing both simultaneously is often difficult in practice because it increases the lead time to produce the end-product, something that Project Managers don't seem to grok. You can successfully do both if you alternate between them, spending substantial chunks of time immersed in one role or the other at any given time.

Developers can be reasonably objective when it comes to writing small, self-contained unit tests and some amount of test automation, but beyond that, where serious QA begins, developers are generally too much in love with their creations to distance themselves from them and view them objectively.

You can see this pattern with authors who become (commercially) successful. The publisher, whose most valuable service is editing the author's work, starts becoming too deferential, causing the author's work to become muddled and flabby. A good editor is often the only one standing between mediocrity and greatness...