There used to be a time when one could get to understand how things worked. For example, back in the late 80's I had a Commodore 64 and after spending many hours pouring over Kernal ROM disassembly books I got pretty familiar with how it ticked. I even had a hefty 1541 drive disassembly reference book as I was trying to figure out how to write turbo loaders. OK, so I didn't have the original source, but I had time, and there was only tens of thousands of lines of assembler to understand.
Nowadays, things are radically different. The Linux Kernel is huge: Millions of lines of C and some assembler, tens of thousands of lines change between versions, but we at least have the source (and documentation!). There just is not enough time in the day to get to understand all of it well enough!
Fortunately there are well defined interfaces and well documented sub-systems, but I fear we are in an age were no mere mortal can fully understand exactly what is going on in the kernel. Complexity reigns.
A typical week may see me dipping in and out of different parts of the kernel - looking at wifi, audio and ACPI. Figuring out BIOS interactions and trying to work out the best kernel tweaks to sort out issues. It can be like an intellectual game, figuring out exactly how some parts work or interact with hardware for the first time can be a challenge, but equally, very rewarding too. Some bugs sometimes seem like they just cannot happen, then you get some different insight to how a system is behaving and then you get that "ahah!" moment and everything becomes very clear and you get a solution, and this is very satisfying indeed.
So, in a way, it's good that the kernel is so large, complex and mysterious in places. There is plenty of scope to keep my gray cells busy; always new features to learn about, new twists to negotiate, new things to discover.
I may get nostalgic about the good old days where life was simpler, code was smaller and less of it to comprehend, but less face it, I'd rather be working on the Linux kernel than writing 8 bit 6510 assembler any day :-)