[Once upon a time, he'd have told her tersely that it's to give him peace and quiet for a few days. The thought even crosses his mind. But instead, he's honest:]

Because no matter how much people tell you that this world is not your own, if you do not go out there to see for yourself that your family is not there, then the thought will continue to trouble you. It will cause you distress until such a time as you are able to get out there to see. Traveling there immediately will cut that period of anxiety to a bare minimum.