I came across these two sentences while reading F. Paul Wilson - Gateways:
But what Jack really seemed to enjoy most was reading far-out fiction and watching old sci-fi and monster movies. His father had worried about Jack, urging him into more social activities. ...
I cannot understand why the author didn't use just "worried" instead of "had worried". I don't think the author wanted to say that the father's worrying was before the period in the past in which Jack seemed to enjoy. What exactly was emphasized by "had worried" and what difference would have been if the author had used just "worried"?