12:46. V’etzem lo sishbe’ru bo. The Chinuch in #16 has a long discussion ahl derech hamussar here about how mitzvos should make you a better person. Specific to this mitzvah, he says that “It is not dignified for royalty and the mentors of mankind to grind bones and break them like dogs and it is not fitting to do so for anyone other than the poorest of the people and the starving.”
R Shmuel Rozovsky of Ponevezh (brought in the Itturei Torah) stressed the importance of good manners, and coarsening effect of its opposite. He brings that Reb Isser Zalman brought a group of talmidim to the Alter to hear divrei mussar and to talk in learning, and afterwards the Alter said nice things about all but one of them. He told Reb Isser Zalman that nothing would come of that boy. R’ Isser Zalman was surprised, because davkeh that boy was an illui. The Alter told him that some sugar spilled on the table, and that boy licked his finger and stuck it into the sugar and put his finger into his mouth (in the presence of the Alter.) A boy that can do that has no future in dvarim shebikdushah. The story goes on to say that the illuisheh bochur did become the rav of a city, but for some reason he had to leave the position, and he became a lawyer, and eventually went to jail for fraud.
It’s a nice story and nice mussar, but I wonder how true it is. A man comes to the Olam Ha'emes after 120, and they do the checklist. "Shomer mitzvos, 100%; kovei’a ittim, yes, well done!; baal chesed, oh yes indeed; and an honest businessman to boot. But oy, your manners were terrible. Go to Hell.” This doesn’t seem reasonable to me. Also, remember the Gemara in Kiddushin 71b that says that hamashtin mayim arum lifnei mitaso deserves to go into galus, and Rav Avahu snorted and said that doesn't make any sense- just because he's a boor doesn't mean he deserves to go into galus. Certain oriental societies have exquisite manners ("when you first meet a business contact, bow slightly and present your card with both hands...") , and those same fine people will vivisect you without qualms. I've always felt that highly refined social behaviors are a warning flag of a hidden and just-barely-latent hostility. I personally know some people who would make Emily Post proud and who wouldn't offer you a job if you were starving- a warm smile and a sympathetic back rub, maybe, but not a job- and others who would do anything to help you but whose manners would make a hungry man lose his appetite.
But the fact remains that even though a mashtin lifnei mitaso might not deserve galus, Hashem does hate him, as the Gemara in Nidda 17a says. And, of course, there is an entire masechta of braisos called Derech Eretz, which sets forth rules of behavior, such as who should eat first, not to gulp your drinks (although I have to admit that at a Renaissance Faire I attended on a very hot day, the person who drew me a beer looked away for a few seconds to get my change and saw my empty twenty two ounce flagon and thought she had forgotten to fill it; I think the rules are different at Renaissance Fairs), and so on. So there you have it. Chazal are telling us that a concern for the feelings of your fellow man even in such epicene concerns as table manners, and dignified behavior- even when one is alone- are associated with spiritual refinement, and that boorishness is inconsistent with spirituality. Irrespective of what pshat is, this is the reason the Tiferes Yisrael learns that דרך ארץ in the Mishna in the third perek of Avos means nimusim and mussar, and that it is listed among the essentials of a fully realized Jewish life: