Despite Sirius' firm belief that he was romantically involved with a slice of crusty white (he wasn't), Peter could not deny that he indeed, loved toast (in a platonic way, you perverts).
He loved toast more than anything in life itself. When his parents cast him away to his Granny's warm cottage, he would always be fed a few slices of not brown but not completely white toasted bread, pooling in creamy butter.
And from then on, toast would always be his comfort, his private joy, his guilty pleasure.
Blimey, that sounded so wrong.
However, he did not tolerate when his friends stole his toast, something they did rather frequently.