The Torah tells us that when בני ישראל traveled towards Mount Sinai, they were lacking in water. They reached a location called מרה. This hints that the water found was bitter and not drinkable.
Hashem tells Moshe Rabbeinu to throw a tree into the water, in order to sweeten it. The symbolism of this event was significant.
The thirst for water represented a desire to already receive the Torah. The tree symbolized the עץ החיים, the tree of life. The water is מים חיים, living waters.
Evidence of this is that the incident ends with the words, שם לו חוק ומשפט, that certain Torah laws known as חוק, Mitzvot without reason, and משפט, understandable civil laws, were given at that time.
As a result of this incident, Moshe decreed that Jews should not go three days without Torah. Therefore, we read the Torah on Monday and Thursday and on Shabbat.
It was Moshe who also decreed sitting Shiva for a mourner and שבע ברכות for a newly married couple. Such decrees were meant to improve Jewish life.