I know this might not resonate since it's Severus/Harry but I'll leave it anyway.

jadzialove wrote a Snarry Christmas Carol in 2005 or so called Humbug. The line that always stays with me, which comes toward the end, is Severus saying this to Harry:

"However, you have, for some unfathomable reason, chosen me, and I shall spend an eternity endeavoring to be worthy of it."