They base their vegetarianism on the fact that the first diet commanded by God in Genesis was a vegetarian diet: "God also said, 'I give you all plants that bear seed everywhere on earth, and every tree bearing fruit which yields seed: they shall be yours for food.'" (Genesis 1:29). The ultimate desire of God is for a world like that in the Garden of Eden, where humans and even animals are all vegetarian:
"The wolf shall dwell with the lamb, and the leopard shall lie down with the kid, and the calf and the lion and the fatling together, and a little child shall lead them. . . . They shall not hurt or destroy in all my holy mountain; for the earth shall be full of the knowledge of the Lord as the waters cover the sea." (Isaiah 11:6, 9)
Compassion to animals is part of Jewish teaching. Animals, as well as humans, are to be rested on the sabbath (Exodus 20:10), one has an obligation to relieve the suffering of animals (Deuteronomy 22:4, Exodus 23:5), and "a righteous man cares for his beast" (Proverbs 12:10). God himself cares for animals, for "his tender care rests upon all his creatures" (Psalms 145:9).
Finally, Proverbs 23:20 advises, "Be not among winebibbers, or among gluttonous eaters of meat."