Billy is able to display a flippant attitude to death and mortality because he has been influenced by the Tralfamadorians, a strange race of aliens who bring him back to their planet to exhibit him in a zoo. Constantly surrounded by death, bloodshed, and destruction, Billy has to figure out a way to deal with life's many horrors. Fortunately, the Tralfamadorians are on hand to give him a much-needed sense of perspective. According to their unique philosophy of time, time exists all at once. So if someone is dead at moment, they're alive at another.

This gives Billy a sense of peace when it comes to confronting dead people. Armed with this Tralfamadorian insight, he's able to deal with death more effectively, almost aloofly, as if it's really no big deal. Because of this, whenever he comes across a dead body, he simply shrugs his shoulders, says, "so it goes" and leaves it at that.

He knows, or thinks he knows, that though such people may be dead at this precise moment, they're alive at others, and so there's no point in getting too down or upset. Every death, like every life, is somehow related in the overall scheme of things. This realization gives Billy a sense of security, the comfort that there's some kind of cosmic purpose to all of this.

On one hand, the answer to this question is straightforward. The narrator tells his readers early on in the story that he says "so it goes" after he hears about the death of some person.

Now, when I myself hear that somebody is dead, I simply shrug and say what the Tralfamadorians say about dead people, which is "so it goes."

"So it goes" is said a lot in this book because a lot of people die. Billy learned the phrase from the Tralfamadorians. It is what they say about dead people. The phrase is equivalent in many ways to the modern-day phrase "it is what it is." "So it goes" is a fairly emotionless phrase about death, and that reflects their attitude about death. A Tralfamadorian sees death as a current condition of that person's entire life. Death might be a "bad condition" at that moment, but the Tralfamadorians can see life's different moments at the same time; therefore, they can see all of the other times when that dead person is not dead and doing just fine. The death is a small moment of an entire life, and that moment is shared by everybody, because death is universal. Death, to a Tralfamadorian, is what it is. It's a small moment, and that's how it goes. The narrator has tried to adopt that kind of view, and whether or not he is successful with it or is making some grander satirical statement is up for debate from reader to reader.

"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 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.

"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.