The past perfect is strictly a referential tense that places something further in the past than some subsequent event. Without reference to that event, there is no reason to use the past perfect.
John had just put down his book when he heard the crash in the kitchen.
First, John put down the book, then he heard the crash. Without the crash that came later, John's putting down his book is just some random event in the past that would be expressed in the simple past tense.
Now there is a construction that looks like the past perfect but isn't:
If I had known then what I know now, things would have turned out differently..
This is the subjunctive/unreal condition that talks about something in the past that did not occur. The result clause talks about what might have happened if it had.