I completely agree. Trauma does things to a person... and Nicky was a reminder of not only what happened in Vietnam, but what had happened throughout their childhood. On some level, I am sure Jack thought that Nicky's attempts at communication were a way to get back into his life and therefore the cycle of cleaning up after Nicky would begin all over again. But Jack had a family, a good job, a good life as he said, and it's hard to have a good life if you're constantly dealing with someone else's baggage and mistakes, plus all of the baggage it would bring up for Jack. He had to choose, as Nicky said, his life BEFORE the war and his life AFTER the war.