In most cases there needs to be a good reason to use a perfect tense -- that you want to call attention to the relationship between two events, or that you need to explain that one action happened before (or precipitated) another event. Otherwise the reader expects the relation to have some significance.
I counted / had counted the money before it was locked in the vault.
"I counted" simply suggests that one event happened, and then the next event happened. But "I had counted" implies that there is some significance to counting the money, in relation to it being put safely away, for example, if I'm being questioned because some of the money is missing.
As FumbleFingers' comment says, in this context there is no reason to connect the events in a significant way. Something happened when Susan left. However, the writer does use the past perfect progressive "had been arguing" to create significance between the arguing and the leaving.
Before I met Sylvia, I led a nomadic lifestyle -- I wandered around from place to place, and never thought about the future.
So far we're just talking about the events of my life, A happened then B, then C, etc. But then I can use the perfect to draw attention to what was going on immediately before the significant event I want to talk about:
I had been living in Amsterdam for three months, working odd jobs, when, one day, our eyes met at a small cafe in the red light district.