This is going to be unsatisfying, but the only thing that works for motivating me to understand a piece of kernel code has been wanting to change it and having to learn exactly how it works to achieve that.
If you just want to learn more about how the kernel fits together, reading http://lwn.net/Kernel/LDD3/ (Linux Device Drivers, freely downloadable) is a fine start.
This is the way the Linux kernel was introduced to us in Uni, and I really think it is the best way. Set a goal like "write a simple device driver", or (my choice) "implement a new concurrency primitive", and work towards that. Trying to do something like adding P/V system calls will teach you a lot about how the kernel operates; enough to give you a good starting place to find something new about it to learn.
If you just want to learn more about how the kernel fits together, reading http://lwn.net/Kernel/LDD3/ (Linux Device Drivers, freely downloadable) is a fine start.