Why does the author continually use "so it goes"?
"So it goes" is a nod to the existential nature of Kurt Vonnegut's life philosophy. Whenever someone (or something) dies in the novel, "so it goes" is Vonnegut's automatic mantra. There is nothing a person can do about death - to happens to us all.
Because the novel's main focal point is the chaos caused by the allied bombing of Dresden, Vonnegut is obviously trying to make the point that war is bloody awful, but also inevitable. When he tells a friend that he's writing an anti-war novel at the very beginning, he's told he might as well write an anti-glacier book. In other words, war will happen; people will die. So it goes.
Billy Pilgrim dies in the book, as do all the other main characters. Even the champagne dies on his bitchy daughter's wedding night. The world is blown up by aliens; they know it happens, but do nothing to stop the fact that it happens. It has always happened. So it goes.
In the existentialist philosophy, people are encouraged to accept the consequences of their behavior, and to acknowledge that sometimes events are beyond their control. "So it goes" mirrors the plaque Billy keeps in his office (the serenity prayer). People should change what they can, but accept that some things are beyond their control. Death is often something that people cannot control or avoid, especially with the backdrop of war. We have to accept it and move on or be terminally stuck in the grief of death.
"And so it goes" is a refrain that is repeated after deaths in Vonnegut's book. And on a first reading may seem to be a part of an argument that there is nothing that anyone can do to stop death or war. It is important, however, to realize that the book is a satire, that looks critically at the apathetic position that it describes.
The Tralfamadorians accept such a position, and Billy Pilgrim seems to as well, but neither of the aliens, nor Billy Pilgrim are presented as heros or examples for readers to follow.
The hero of the Tralfamadorians, remember, is Charles Darwin. Billy Pilgrim's story of apathy ends in an apocalyptic scene in which civilization is destroyed. The birds don't seem to care but we, as humans, are meant to.
Thus, although, "and so it goes" makes it sound like Billy Pilgrim doesn't care, when we read the phrase as a part of a book of satire, we are meant to read its tone as that of a lament.
"So it goes" is Vonnegut's cue that another life has passed, or is passing. It is part of Vonnegut's genius that he is able to both poke fun at our mortality (I cannot recall how many times "so it goes" is used, but it's so frequent that it becomes darkly funny) but also to remind us of, sadly, of what "blips" we all are on this planet.