Four things were said about bread: one should not leave raw meat on bread; one should not pass a full cup over bread; one should not throw bread; one should not rest a full tray on bread. Ein Ayah:
The dignity with which we treat bread is an indication of our treating life as holy and worthy of honor. Since human life has a lofty purpose, it is proper to show respect to life, including by showing respect to that which is the main sustainer of life, namely, bread. Bread also deserves extra respect in that it is a sign of mankind’s advantage over other species. Firstly, man’s wisdom increases when he eats bread, as Chazal said: "A baby does not learn to call, ‘father’ and ‘mother’ before he has tasted grain" (Berachot 40a). From the perspective of man’s wild side, he is willing to eat meat like carnivores, and we even find cannibalism amongst primitive tribes. One sign of wildness is eating raw meat, and we find that Chazal lambasted the Babylonians for this (Menachot 99b).
Eating bread is a sign that man has raised himself above carnivorous animals, by recognizing our higher innate level of being formed in the image of Hashem and being similar to Him in that we are able to do that which is straight and good. That is why we chose our main sustenance to be from the vegetable realm. If one is not fully sustained by eating raw vegetables, then he is smart enough to know how to turn grain into bread, about which it is said that it "satiates the heart of man" (Tehillim 104:15). When mankind is fully healed of the damage inflicted by the snake, we will not need to eat meat anymore.
Eating meat was reluctantly permitted because of the weakness of the body, which came from the weakness of the soul that arose when man lost his morality and closeness to his Maker. The Torah describes eating meat as something that comes when "one’s spirit lusts for meat" (Devarim 11:20). Chazal further looked down upon ignorant people who eat meat, as it was permitted for scholars who need the nourishment to have the strength to seek Torah knowledge. Since that brings light to the world, it is proper that animals do their part towards that lofty goal. Animals’ sacrifice is reminiscent of soldiers’ self-sacrifice at war for their countries and those who undergo punishment so that people’s hearts will be straightened out. When this will not be necessary, scholars will not eat meat either, and the pasuk, "You will eat your bread to satiation" (Vayikra 26:5) will be practiced.
Let us analyze the gemara’s practices in this light. One may not place raw meat on top of bread because bread represents cultured eating and raw meat represents primitivism. While it is cultured to use other foods to enhance the bread, overdoing this can bring one to excesses, of which the Torah especially noted intoxication. That is why one does not pass a full cup over bread, as being full is usually a sign of a drink drunk for enjoyment, not necessity.
In order to show respect to life, one should not throw bread, an act that represents disregard for that which is necessary for life. However, the reason to live is to fill oneself with wisdom and knowledge of Hashem, not to enjoy sensual pleasures. Therefore, one should not lean a full tray on bread. That act represents that life (=bread) is there to support a life full of pleasures (the pot full of tasty foods). Rather, bread exists to enable one to base Torah and wisdom on the strength it provides.