Lyric discussion by TheF16Falcon 

Cover art for The Unforgiven II lyrics by Metallica

My interpretation probably doesn't matter at this point because so many others have posted already, but I'll give it a go anyway. I feel the lyrics describe an introverted guy who finally finds a girl that understands what it means to be lonely. The "door" spoken of in the song is the guy's soul, and the girl is the only person who has ever been able to make it "crack open." She dies ("The door is closed, so are your eyes"), and he goes on waiting for death, alone again, because she was the only one with the "key" to his soul. I can almost relate to this song...I've yet to meet that girl.

@TheF16Falcon I agree, except I don't think she actually dies. I think it's a metaphor that their inability to forgive each other and their pasts and move on has killed her inside. So he finally finds a way to see the sun and a way out of it and he plants the key (or the seed) inside of her too, because he sees that she is also unforgiven and all they both want is to forgive themselves, and each other, and move forward into happier times. I think he's finally figured out his way out and is trying to lead...

@TheF16Falcon i was under tthe interpretation he killed her