Minhagim have become incorporated into daily practice, like law.
Reprinted from The Jewish Religion: A Companion, published by Oxford University Press.
Originally, the term minhag, "custom" (from a root meaning "to follow," i.e. that which people follow) referred to a practice about which the law was unclear, perhaps where certain details were the subject of debate by the legal authorities. When it was observed that the people followed a particular interpretation or ruling, the practice of the people was decisive and this practice acquired full legal status. As the Talmud (Berakhot 45a) puts it: "Go out and see what people actually do."
Development of Minhagim
But in the Middle Ages, especially in Germany, the people followed certain practices for which there was no support in the law; sometimes, such practices were adopted from the customs of the peoples among whom Jews resided. At times, the rabbinic authorities were suspicious of this kind of folk custom, but when it became too deeply rooted to be eradicated, even this type of custom was also incorporated into Jewish law, on the principle, evidently, of "if you can't beat them join them," and a new Jewish interpretation was given to the custom so as to render it innocuous.
A good example is the practice of breaking a glass at a marriage ceremony. It seems that this practice took root among German Jews, the Ashkenazim, because they were enamored of a similar practice they observed among German folk which was intended to trick the demons into believing that a catastrophe rather than a celebration was taking place; the demons would then leave the couple alone and do them no harm. The practice was eventually accepted by the Rabbis; the Jewish interpretation given to it was that it reminded the couple, on their happy day, that they should reflect on the destruction of the Temple, in other words, they should be aware, even on this day, of the sufferings of their people and not selfishly ignore them.
There was constant tension over the adoption of new customs. On the one hand, there was the need to cater to the masses and keep them faithful, but on the other hand, the pagan origin of some customs was too blatant to be ignored. This tension resulted in two contradictory sayings. One saying has it that the custom of Israel is Torah, that is, custom has the binding force of Jewish law. Against this is the saying that when the letters of minhag are transposed they form the word gehinom, Gehinna (hell). But, generally speaking, more of the folk-customs were accepted than were rejected.