This is the positive lesson from the plague in which water was transformed to blood. Water, which is cold, represents the coldness and apathy in divine service that stems from the influence of the concealment of G–dliness in the world. In contrast, blood, which is hot, represents passion in our divine service. A Jew should transform himself from apathy to Hashem into passion for Hashem.
At a chassidishe farbrengen we strive to heat ourselves up in this way. Moreover, we imbue staying power into this passion. We inspire ourselves with the inner strength not to be affected by the outside world at all—not only do we not fall in sin (G–d forbid), but our passion for Hashem and all things holy will last unchanged.
Along these lines, I heard Rabbi Shabtai Slavaticki of Antwerp, Belgium relate:
Soon after I arrived at the Chabad Yeshivah in Kfar Chabad, I asked the mashpia, Reb Shlomo Chaim Kesselman, why chassidim wish one another l’chaim on mashkeh (alcohol). He explained that mashkeh is different from other liquids. When other liquids are placed in a freezer, the cold atmosphere around them makes them freeze. In contrast, when mashkeh is placed in a freezer, it remains a liquid.
Likewise, at a chassidishe farbrengen, chassidim make a point of wishing l’chaim on mashkeh, because the nature of mashkeh reminds a chossid that he should never allow himself to be affected by his surroundings and “cooled off” from his passion in serving Hashem.