Sirius was prepared for anger. He had steeled himself for hatred. He had anticipated loathing.
None of that would have hurt as badly as Remus's gentle touches, his sweet forgiveness. Every day, when the son rose, so did Sirius's shame and guilt. If Remus could have been a decent bloke and mustered a little anger, Sirius felt he could possibly even have moved on from the betrayal of trust that had killed his best friends, left Harry an orphan, and abandoned Remus friendless.
Remus, gentle spirit that he was, wounded Sirius most of all in kindness. Sirius spent every day trying to be someone worthy of forgiveness.