The Jewish word for "blessing," ברכה, comes from the same root as the word "wellspring" (בריכה). The imagery of the wellspring shows how bountiful and renewing a blessing can be, and how it brings additional good to the world. The word ברכה also has the same root as the Hebrew word "knee" (ברך). The knee imagery shows that we metaphorically bend down to give recognition and show appreciation for G-d's kindness. Combined together, a blessing is the Jewish way we thank G-d for all that He provides. With that out of the way, why 100 blessings? Why not 5, 20, or 250? What is special about the number "100?"
- In Deuteronomy 10:12, Moses tells the Jewish people, "What (מה) does G-d ask of you?" In the Talmud (Menachot 43b), R. Meir tells us to read the word "what" (מה) as "one hundred" (מאה), i.e., the verse reads "100 [blessings] G-d requires of you." This is to instruct us in the mandatory 100 blessings a day in order to help us cleave to, love, and fear G-d (Orach Chaim 46:3).
- Midrash Rabbah (Leviticus 18:17) said that in the time of King David, there was a plague that was killing 100 people a day. King David's anecdote? 100 blessings a day. Afterwards, the plague stopped.
- In the Book of Exodus (38:27), there are 100 sockets used in the Tabernacle, which correspond to the 100 blessings (Ba'al HaTurim). Functionally speaking, sockets hold other objects in place, whether we are talking about eye sockets, tooth sockets, or light bulb sockets. We can take this metaphor that blessings, much like the sockets on the Tabernacle, hold our relationship with G-d in place.
- In his commentary Reishit Chochma, sixteenth-century Sephardic rabbi Eliyahu de Vidas used the Zohar to use gematria (Jewish numerology) on the word צדיק (righteous person). He took the letter ק, which has the value of 100 in gematria, from the word צדיק, and opined that part of being a righteous person is saying a hundred blessings a day.