I promised to protect you, Potter

Dumbledoor's soul lingers so that he might bring Snape back from beyond the grave. Severus explained to Harry that he had only done as the ex-headmaster requested and that had there been another way he would have done it. All Severus ever did was try to protect the son of the man who had put him through so much in their Hogwarts school days. Rated T just to be safe. Mostly canon.