The art of country music songwriting is like no other art. Instead of relying on clever innovation or pop catchiness, it relies on simplicity: great country songs are the ones that everybody wants to sing. The gold standard for country song writing is Hank Williams: his songs are so timeless and covered so widely that you might not even know how often you've heard them. They have been adopted and adapted by countless singers, and sound good coming out of just about anyone who has talent.
Willie Nelson achieved the same effect with "Crazy." It is lyrically simple, but its theme—impossible love—is transcendent enough to make the song as relevant today as it was then. He also manages to fit the straightforward lyrics into a complex, showy tune, well suited to the likes of Patsy Cline. In fact, "Crazy" fits her like a glove, so much so that the song's words and themes will probably forever be associated with her.