"Rejoice in the Lord always. And again I say, rejoice." Philippians 4:4 And Paul says again, "Rejoice in the Lord always" 1 Thessalonians 5:19

The Amplified Bible says to 'delight, gladden yourself in Him, be glad-hearted continuously.'

Believers rejoice--
In God's salvation and we can say with David:
"O Lord, in Thy strength the king will be glad, And in Thy salvation how greatly he will rejoice." Psalm 21:1

In God's protection, David's song when he was in the wilderness:
"For Thou hast been my help,
And in the shadow of Thy wings I sing for joy." Psalm 63:7

In God's blessings:
"That I may see the prosperity of Thy chosen ones, That I may rejoice in the gladness of Thy nation, That I may glory with Thine inheritance (Thy people)." Psalm 106:5 (Margin note)

In the Lord Himself:
"Yet I will exult in the Lord,
I will rejoice in the God of my salvation." Habakkuk 3:18

The cause of the Believer's rejoicing is-- In God's Word:
"Your words were found, and I ate them,
And Your word was to me the joy and rejoicing of my heart; For I am called by Your name, O Lord God of hosts." Jeremiah 15:16

In the Believer's assurance of who Jesus is:
"He (Jesus) said to them, 'But who do you say that I am?' Peter answered and said, "The Christ of God." Luke 9:20

In salvation (the parable of the lost sheep Vs. 4-7):
"And when he comes home, he calls together his friends and neighbors, saying to them 'Rejoice with me, for I have found my sheep which was lost!' 'I say to you that likewise there will be more joy in heaven over one sinner who repents than over ninety-nine just persons who need no repentance.'" Vs. 6-7

In persecution (Peter and the other apostles with him after preaching Jesus the Christ and being put in prison, Acts 6:29-42):
"...when they had called for the apostles and beaten them, they commanded that they should not speak in the name of Jesus, and let them go. So they departed from the presence of the council, rejoicing that they were counted worthy to suffer shame for His name." Vs. 40-41 However they did not stop as we see in verse 42 says, "And daily in the temple, and in every house, they did not cease teaching and preaching Jesus as the Christ."

In reunion of Believers (for example, concerning Epaphroditus):
"For indeed he was sick almost unto death; but God had mercy on him, and not only on him but on me also, lest I should have sorrow upon sorrow.
Therefore I sent him the more eagerly, that when you see him again you may rejoice, and I may be less sorrowful. Receive him therefore in the Lord with all gladness..." Philippians 2:27-29

In Christ's return:
"But rejoice to the extent that you partake of Christ's sufferings, that when His glory is revealed, you may also be glad with exceeding joy."
1 Peter 4:13

Therefore, we sing
"And rejoice before Him and rejoice before Him.
And rejoice and rejoice before Him."!!!

Thank you most Holy Almighty God for Your loving kindness and graciousness to us! We have every reason to rejoice always whatever the circumstances for we are Yours and You are in control. Hallelujah.
Amen