It is written that a snake's venom is exceedingly hot. This alludes to a condition in which a Jew becomes so wrapped up and roused by the heat of the material world - namely the excitement in the mundane - that his spiritual inspiration is compromised.

Concerning the scorpion it is stated that his venom is cold, connoting a more insidious situation than that of a snake. When one is enthused and excited - albeit in matters relating to the mundane - at least it is a sign of life. He can then channel that enthusiasm to holiness. However, when one is cold and uninspired - a sign of the opposite of life - it is considered far worse.

This will serve to explain the mishnah: "If one encounters a snake around his ankle during the Shemonah Esreh prayer, he is not to interrupt, whereas when a scorpion is near his ankle he is to interrupt [his prayer] and deal with the impending danger." (Berachos 5:1)

When, in the midst of prayer, one is overwhelmed with passion and excitement for worldly things (comparable to the heat of the snake's venom), he is not to interrupt his prayer. Since he possesses the proper tool for spiritual growth, he must merely learn to channel it in a positive way.

On the other hand, when one is besieged during prayer by a sense of coldness and lack of enthusiasm (comparable to the venom of the scorpion), he must interrupt his prayers, since this indicates that his service to G-d is totally inappropriate and must be reconstructed anew.

Likkutei Sichos, vol. 2, p. 374