This is the challenge of modern software development. When I first started my career, I spent two months looking for a random crashing bug in a video game due to misplaced free(); this was in code that was 100% ours. Now, there likely isn't an application in existence that does anything interesting where 100% of the code was written for the app itself. As such, you are no longer debugging your own code, but other people's as well.

Vernor Vinge coined the idea of a software archeologist, people who could sift through layers of code, all the way back to the beginning of time (Jan 1, 1970 and Unix, of course), to understand how systems work and to make modifications to them. We aren't that far from that point; already, there are people who seem to specializing in digital spelunking to find and fix bugs in these underlying layers.

While I love building new code, some of the most satisfying moments in my career have been when I've gone back through somebody else's code, untied it (oh, you built a polymorphic type system including class initializers and destructors in a decidedly not object oriented language? Cool.) and fixed it. There's something interesting about getting into somebody else's head and seeing how they approached the problem, then finding out where they were wrong.