Good question and thanks for trusting me with the answer.
We of course don't wish folks a 'happy new year'.
We wish them a Shana Tovah, a good year.
So where does sweet come into play? ["u'metuka"]
Well first, we need to remember that there are 4 Jewish new years.
The one this Monday, is on the first of the 7th month.
It is the New Year for not Jews, but for humanity, because Adam and Eve, so we teach, we born, created on this day.
And the Jewish people's New Year is on the first day of our first month, Nissan.
And we celebrate it 14 days later, as Passover, when we were born as a free nation.
And we say then, have a sweet Passover.
So the sweetness got carried over.
The other new years, are first of Elul which is for animals, and first of Shevat, moved to the 15th of Shevat, by Rabbi Hillel, i.e. Tu B'Shevat, which is for fruit growing trees.
Now for sweetness: it is a bit like which came first, the chicken or the egg.
We eat an apple on Rosh Ha Shana. The apple symbolizes Gan Eden (the Garden of Eden), which according to the Talmud has the scent of an apple orchard, and in Kabbalah is called "the holy apple orchard." Isaac commented regarding his son Jacob (Genesis 27:27), "Behold, the fragrance of my son is like the fragrance of a field, which the Lord has blessed!" The Talmud explains that this refers to the scent of an apple orchard, the scent of Gan Eden.
SHALOM and BLESSINGS: