The Siach Sarfei Kodesh says in the name of the Ksav Sofer a most interesting explanation for the prohibition of COOKING the Korban Pesach. In Tehilim 81:4,5 it says,
1) Tiku Bachodesh Shofor, -
2) Ba'kesse L'yom Cha'geinu, -
3) Ki Chok L'Yisroel, -
4) Hu Mishpot L'Eilokei Yaakov.
As indicated, the Ksav Sofer divides these two verses into four sections. Upon taking the first letters of each group, we find the following words:
T-B-S = Shabbos, B-L-CH = Cholov, K-CH-L = K'chal, H-M-L-Y = Milah.
These four words have in common that they represent the characteristic of mercy in a situation that is basically one of stern judgement, "din."
The six days of the week are days of "din." However, Shabbos is the exception. It has the trait of mercy, i.e., Gehinom is cooled on Shabbos. Injury to one's body is generally also a concept of "din." Milah, however, is the exception. It improves and elevates, although it too is an injury. Foodstuffs derived from animals is also "din" in that it requires the killing of the animal to halachically allow for consumption. Milk is an exception in that it does not require killing of an animal.
K'chal, an animal's udder, is also an object that signifies mercy. Meat may not be cooked with milk (Shmos 23,19). An insight into this might be that since the meat represents "midas hadin," Hashem does not want it to be cooked with milk, which as stated above, represents "midas horachamim." An udder, when cooked in its own milk which has not been previously removed from it, may be eaten. The characteristic of the milk, mercy, overpowers the characteristic of the meat, din.
Given the above, an obvious question arises. Why is it permissible to cook and consume meat that is boiled in water? The Ksav Sofer answers that only milk succumbs to the "midas hadin" of the meat, because the milk is also a product of an animal. Water, however, is such a pure and powerful midas horachamim, as it is not an animal product, that it is not negatively influenced by its involvement with meat. The above gives us a new insight into the prohibition of COOKING the Korban Pesach in WATER. The Holy Zohar says that the level of impurity in Egypt on the night of the smiting of the firstborn was so great, that Hashem would not send angels to carry out the plague for fear that even they would become contaminated from the severe impurity that prevailed. In such a negative spiritual environment, even water, the embodiment of that which is pure, could be overpowered by the "midas hadin" of coming in contact with meat. Therefore, cooking the Korban Pesach in water was prohibited on that night. To remember this concept, it has been prohibited for all generations.
