"אבינו מלכנו חטאנו לפניך"
“Our Father our King we have sinned before You.”

QUESTION: Why does the individual say “chatanu” — “we sinned” — in plural?

ANSWER The Gemara (Shevuot 39a) says that all Jews are responsible one for another. The reason for this is that the Jewish people are like one body. Thus, the Jew who sins affects the entire Klal Yisrael. Likewise, when a Jew does a good deed it has a good effect and benefits the entire Jewish people (see Rambam, Teshuvah 3:4).

A passenger on a boat once noticed another passenger drilling under his seat. In astonishment, he bellowed “what are you doing?” The other responded, “Mind your own business. I’m drilling under my seat. I paid for a voyage on this seat.” The man said to him “Fool, don’t you realize that if water comes in under your seat we are all doomed”!

(טעמי המנהגים בשם האר"י ז"ל)