BECAUSE THEY UNDERSTOOD

THANK YOU FATHER.

And all the people gathered themselves together as one man into the street that was before the water gate; and they asked Ezra the scribe to bring the book of the law of Moses, which the LORD had commanded to Israel.

And Ezra the priest brought the law before the congregation both of men and women, AND ALL THAT COULD HEAR WITH UNDERSTANDING, upon the first day of the seventh month

And he read thereof from morning until midday, before the men and the women and those that could understand; AND THE EARS OF ALL THE PEOPLE WERE ATTENTIVE UNTO THE BOOK OF THE LAW.

( They didn't pluck out a scripture and give 8 hours of a rant of their own opinion or private interpretation)

Ezra stood up before a pulpit that was made of wood and set higher as a stage so that all could see him.
Ezra was surrounded on all sides by men who sought to learn of God.

Ezra opened the book in the sight of all the people; and when he opened it; all the people stood up:
And Ezra blessed The LORD, The great God.
And all the people answered, A-men!
A-men, with lifting up there hands: and they bowed their heads, and worshipped the LORD with their faces to the ground.

There were others there Levites who were teachers that caused the men who heard the law to understand the law: and the people stood in their place.

So they read in the book of the law of God distinctly, and gave the sense, and caused them to understand the reading.

(They didn't tell the people what the law meant they helped the people to understand for themselves what was read.)

And Nehemiah, Ezra, and the Levites that taught the people, said unto all the people, This day is holy unto the LORD your God; mourn not nor weep. for all the people wept, when they heard the words of the law.

Then he said to them, Go your way, eat the fat, and drink the sweet and send portions unto them for whom nothing is prepared: for this day is holy unto our Lord: neither be ye sorry; for the joy of the LORD is your strength.

So the Levites stilled all of the people, saying, Hold your peace, for the day is holy neither be you grieved.

And all the people went their way to eat, and to drink, and to send portions, and to make great mirth.

Why did they celebrate at the reading and then understanding of the Word of God?

Because when you listen to some preach it can be grieving.
The knowledge of the wages of sin can be grieving.

But what is it about learning that you are a sinner that can take you from the reality of it's grief and all that it entails to the joy of The LORD?

John 3:16
John 10:10

And all the people went their way to eat, and to drink, and to send portions to those who had none because they had understood the words that were declared unto them.

Faith comes by hearing and understanding The Word Of God.
Romans 10:17
Proverbs 10:13

Love Y'all :)