@Kirin-kun
It's mainly because they are sitting in front of each other in an official match, like they both had to put a lot of work to get there, so Rei starts thinking who put more work than the other and so on. It´s part of Rei's personality, like that time he didn't want to win against that old man who was about to retire. It's not a healthy way of thinking in the middle of the match for sure, just goes to show how inmature he still is.
@miyako19
Even if is not manual labors, Rei's way of living is still way out of the ordinary. By the age of 16, he already had an steady income of money, including savings, with an apparment for himself, and he managed to work and study, while also cooking, cleaning and shopping all by himself. And he has maintened this life for 3 years by now. That's a feat even adults have difficulty achieving.
And by the way, it has never been clear if Rei actually likes shogi. He started playing to impress his father when he was a kid, and then kept playing to run away from reality, and now his life depends on it so he can´t stop, or so he thinks. Like, it's obvious to think he does like it, but he has never said so himself i think, and he always plays because he has to more than because he wants to.