Self-Inventory

Somewhere in the novel Slaughterhouse Five there must be some deeper understanding or meaning for the craziness. The way the whole book jumps back and forth between different events in Billy Pilgrim's life has to have more meaning. It could easily be explained by the author going mentally insane or senile some kind of disease like that but other than that it still doesn't make sense to me. There is no emotion in the entire book other than when Billy Pilgrim sees the abused horse, not a tear when his wife died, no sadness when his friends and soldiers were killed, nothing. The recurrence of "So it goes" after every death or bad event symbolizes the way he felt about almost everything. Why did the author pick those words? Kurt Vonnegut dealt with a lot throughout his life but why was the whole theme of the book negative? Of all the different characters that could have been chosen why Valencia? Why the one woman who no one wanted to marry and was described so poorly? What was the point of adding the gruesome scene with the killing of the dog?