Slaughterhouse-Five: "So it Goes"

"After Edgar Derby, the high school teacher, was shot in Dresden later on, a doctor pronounced him dead and snapped his dogtag in two. So it goes." p.92

This is probably the one hundredth time "so it goes" has been written so I figured I'd give it a well-deserved blog. Obviously, with this being repeated so often, it is connected somehow with the theme. So far from the story, I haven't been able to identify a key theme, but I really think this reoccurring phrase has some connection. It really captures the laid-back atmosphere within the progression of the story. Everything seems to be happening almost without care. Billy seems to have no control on the actions in his own life as he drifts through time. The phrase "so it goes" influences the reader to come to understand things in the way Billy perceives them. It implies that everything must be taken as it is because it simply does not change. I hope this analysis is on the right track. I'm interested to see what other's thought of the repetition of this phrase.