Nehemiah 8:10b - The Joy of the Lord is your Strength.

Neh 8:10b (NIV) Do not grieve, for the joy of the LORD is your
strength."

The sequence in chapter 8 is striking: intellectual response to
the Word (vv. 1-8), emotional response to the Word (vv. 9-12), and
volitional response to the Word. [Bible Knowledge Commentary]

The better we understand the word of God the more comfort we
shall find in it; (Matthew Henry's Commentary)

Knowledge of the Word of God can create the conditions for
dramatic spiritual renewal.... instruction in the Word of God has
frequently transformed lives, led to personal salvation, brought comfort
and joy, produced tears of conviction, and reawakened commitment to
God's purpose. [Disciple SB]

There is joy in hearing the Word, but greater joy at obeying it.
[Wiersbe Expository Outlines]

Sorrow for sin must not hinder our joy in God, but rather lead
us to it and prepare us for it. (Matthew Henry's Commentary)

Holy mourning makes way for holy mirth; those that sow in tears
shall reap in joy; those that tremble at the convictions of the word
may triumph in the consolations of it. (Matthew Henry's Commentary)

The joy of the Lord is your strength. Let it not be a carnal
sensual joy, but holy and spiritual, the joy of the Lord, joy in the
goodness of God, under the direction and government of the grace of God,
joy arising from our interest in the love and favour of God and the
tokens of his favour. (Matthew Henry's Commentary)

Holy joy will be oil to the wheels of our obedience. (Matthew
Henry's Commentary)

Religious joy, properly tempered with continual dependence on
the help of God, meekness of mind, and self-diffidence, is a
powerful means of strengthening the soul. In such a state every duty is
practicable, and every duty delightful. In such a frame of mind no man ever
fell, and in such a state of mind the general health of the body is
much improved; a cheerful heart is not only a continual feast, but
also a continual medicine. (Adam Clarke Commentary)

The joy of the Lord will arm us against the assaults of our
spiritual enemies, and put our mouths out of taste for those pleasures
with which the tempter baits his hooks. (Matthew Henry's Commentary)

Psa 28:7 (NIV)  The LORD is my strength and my shield; my heart
trusts in him, and I am helped. My heart leaps for joy and I will give
thanks to him in song.

It seems strange that a man in prison would be telling a church
to rejoice. But Paul's attitude teaches us an important lesson: our
inner attitudes do not have to reflect our outward circumstances. Paul
was full of joy because he knew that no matter what happened to him,
Jesus Christ was with him.... If you haven't been joyful lately, you
may not be looking at life from the right perspective.... Ultimate
joy comes from Christ dwelling within us. [Life Application SB re
Phi.4:4]