Why Will and Lyra?
When I first read the His Dark Materials trilogy, I immediately loved these characters. I became completely immersed in their tale and thought it was one of the most beautiful things I have ever read when Pullman described their love for one another. Their story is tragic and those tend to be the ones we remember most. I have rarely cried as much as I did when they had to be seperated, never being able to fully realize and explore their love.
There is one thing that makes it sadder for me. I can't remember the page, so forgive me for not getting the quote just right, but I remember a scene when Lyra is watching Will and she thinks to herself that when he falls in love with someone, it will be forever. So I've always had this mental picture of Will growing old, never moving on from Lyra, wishing to remain forever faithful. It's absolutely heartbreaking.
Why Into the Dark?
I took it from the Death Cab For Cutie song, I Will Follow You Into the Dark. I took inspiration from the fact that Will and Lyra left open the doorway in the Underworld so the dead could return to Earth for their atoms to become part of the Earth. They agreed to wait for each other, so they could go out into the world and their atoms would cling together and they would never be seperated again. There's also the fact that Will followed Lyra into the Underworld in the first place. Hence, he will follow her into the dark. Make sense? I hope so, because I'm not entirely convinced it does.