So it's interesting to see a counterexample. What led you to go spelunking in nginx?
Also, I have seen so many problems arise from the fact that code is easier to write than to read, so I consciously make an effort to not avoid reading code, and I’m sure that leaked over here.
I used to read source-code specifically to look for security problems. Auditing of code as a small hobby. I found some fun issues over the years.
Reading code has become a habit these days. If I depend upon a library I try to glance at it. If I install a new application I try to have a read of some of it. It stops you from blindly depending on poor-quality software, and sometimes makes you choose an alternative.