The short answer would be that her faith enables Dilsey to maintain her strength. She has, indeed, "seed the first and de last," whether that refers to the Compson children or to a more spiritual truth. But Dilsey is also a version of the Faulkner family's own servant, Caroline "Mammy Callie" Barr, whom, according to Jay Parini's Faulkner biography, One Matchless Time, Faulkner described as "'a fount not only of authority and information, but of affection, respect, and security.'" Faulkner went on to say that she had been "'born in bondage ... a dark and tragic time for the land of her birth,' and 'went through vicissitudes which she had not caused.' Through all of this, she 'assumed cares and griefs which were not her cares and griefs,' accepting whatever trials and trevails befell her 'without cavil or calculation or complaint.'" If Dilsey is the representation of Mammy Callie, who in many ways raised William Faulkner as his "second mother," and to whom he was quite devoted, it's only natural that Faulkner would depict her is a positive light amid all of the disillusion and decay of the Compson family and the South in general.