Completism
Typically, when I get into something in the way of music, I tend to want to hear every note of every recording ever done by said artist.
My latest completist jag - the music of Jorge Ben Jor. Most of what he's done is rare, out of print, and foreign to boot.
The penultimate was The Kinks. Gee, they only had, what... a couple dozen studio albums?
Oh, not to mention Guided By Voices and assorted side projects of Robert Pollard, Tobin Sprout, and the Tobias Brothers (Keene Brothers, Lifeguards, Go Back Snowball, Airport 5, Cobra Verde, Doug Gillard, Acid Ranch, Hazzard Hotrods, etc. etc. etc.). Only about a metric ton of stuff there.
Why don't I ever do this the easy way?