There's no real formula for when inspiration is going to strike for artists. You sometimes have to give it time and work up to making a classic song, and sometimes it comes to you all in one go. Then you also have the kind of songs that almost seem to come together completely by accident.
For as classic as these songs are, most of them came from the band just goofing around in the studio. Whether it was just the vibe in the room or just a little germ of an idea, these are the kind of spur of the moment songs that found their way onto the final tape. Then again, it's never a bad thing to make something that's completely improvised. If anything, these are the songs that capture the kind of ramshackle energy of the sessions a lot of the time, making you feel like a fly on the wall during the recording as you see the band slowly fall into place.
While these songs are played without a care in the world in most cases, they've also been insanely hard to cover as well. When you have the band themselves running on pure adrenaline half of the time, there's no way to really capture that entire thing again. These were a moment in time, and the tape just happened to be running when lightning struck.
10. Nothing Else Matters - Metallica
Looking back on Metallica's Black Album, the thrash legends weren't necessarily trying to write the same type of epics that they made on Master of Puppets. When you listen back to stuff like Enter Sandman or The Unforgiven, this was a much more radio friendly sound that helped establish them as one of the few metal bands that could actually get songs on the mainstream charts. For their beautiful ballad though, James Hetfield's inspiration actually came from mindless noodling.
Since James is the one known for some of the heaviest riffs in Metallica's catalog, the main fingerpicked line of Nothing Else Matters actually came from him sitting on the couch while talking on the phone. With most of the lick being just open strings, Hetfield quickly cut the conversation short and started to add little extensions where he could, like the harmonics that sound like little bells whenever they're played in the song.
Even when it was finished though, James had to be convinced to turn it into a Metallica song, thinking it wouldn't really suit the band's style and would be more of a sellout than what they were already doing. If they were going to make a ballad though, they were going to tackle it properly, bringing in an orchestra that would become the inspiration to do the S&M concerts just a few years later. While there might be a lot of destruction in Metallica's songs, you can't go wrong when you get this personal.