Throw on some Pendulum and I can be sucked into mapping for nearly 2-4 hours easily. Doesn't matter if it's for anything and I don't care if it looks good. Creating maps is all I've ever enjoyed truthfully, ever since I was 4 I've wanted a job just creating environments. When I got a PC for the first time way back when I got sucked into level design incredibly fast. So naturally getting motivation to map is pretty easy for me.
I guess if I wanted to find motivation I'd just run around a few maps in TF2, google some theme idea's, then blast a nice song.
I found Beethoven to also be great mapping music.