Answer:
Nick feels this way because he had returned from WWI
Step-by-step explanation:
After being in WWI Nick does not feel comfortable at home as he says in these lines "instead of being the warm center of the world" everyone back home knew him as the man who went to war, a very important historical event and he did not want to keep that in his life all the time and as home did not home as it anymore he started to feel overwhelmed and disappointed.