“They came to Marah (mar in Hebrew means bitter) and could not drink the waters of Marah for they were bitter; therefore, they named it Marah”. (Exodus 15:23)
The Baal Shem Tov explains that the word “they” is speaking not about the waters, but about the Israelites. Indeed, people who are jaundiced and negative by disposition tend to experience much of life through that filter.
Several verses later, God instructed Moses to cast a tree into the waters, and they became sweet (v.25). Rabbi Abraham Twerski suggests that the Torah is referred to as a Tree of Life that brings happiness to those who embrace it (Proverbs 3:18). Torah has the capacity to change us on the inside and become sweeter people.