The Torah states (in this week’s parsha) “And Hashem formed man from the dust of the ground, and He blew into his nostrils the soul of life, and man became a living being” (Bereishis 2:7). Targum Onkles translates the verse’s last phrase as “and man became a speaking spirit.” It is the power of speech which defines man’s essence and distinguishes him from other creatures.
Thus does Scripture state “One who guards his mouth and tongue, guards his soul from tribulations” (Mishlei 21:23). Shmiras haloshon is singled out because speech is man’s essential quality. Impairment of this power deprives the soul of its essential quality in the next world and is the source of its ultimate tribulation. ~ The Chofetz Chaim