"Lithium" (Nirvana)

I loved the song "Lithium" from the moment I heard it.  Its shifting dynamics.  The almost-jazz feel of the verses set against the crunch of every chorus.  Kurt Cobain's ardent cries of I'm not gonna crack.  It was just the perfectly crafted rock song that didn't feel crafted at all.
As the third, formidable single in a row released from Nevermind (1991), I think it signaled to the world that Cobain wasn't just some lucky bastard who'd scored a major label recording contract.  He had intelligence and a real talent for writing damn good songs.
"Lithium" was mostly inspired by Cobain's stint, staying with his friend Jesse Reed's born-again family when he was a teenager.  Cobain's dad had kicked him out of the house for bad behavior, and the Reed family took him in.  During his stay, he became a frequent churchgoer but became disillusioned in time with religion.
In the song, the protagonist has lost his girlfriend (apparently, she's died), and he's struggling to find the will to stay alive himself.  So he turns to religion to save himself—the metaphor being that religion is a drug, like lithium, that serves to numb the pain of life and gets you through another day.
Although the story in the song is itself fictitious, in real life, Cobain pretty much felt that, if religion worked to keep someone sane/happy/alive, then rock on.