“Always be filled with joy in the Lord. I will say it again. Be filled with joy.”
Philippians 4:4 ERV

“Rejoice in the Lord always: and again I say, Rejoice.”
Philippians 4:4 KJV

Paul is writing from prison. He’s in prison because he’s living out his faith in Christ and fulfilling His calling by telling others about Jesus. He was being persecuted (being punished) for doing the right thing. How many of us wouldn’t be depressed and upset because we’re in prison without blame, for our faith in Jesus, for doing what He’s called us to? (The funny thing is many Christians are depressed while living in a level of luxury that many other people would love to have). If you allow your circumstances to determine joy, your emotions will be up and down like a yo-yo and you’ll be a victim to your environment. But you can be full of joy in spite of what’s happening around you. When times are tough and it seems that all hell is breaking loose, we need joy more than ever because it gives us strength to get through. If Paul could encourage us to rejoice in the Lord always (no matter what) even while he was in prison, then we need to realize that the level of joy that we experience is dependent on our focus – not our circumstances.

You may be going through a really rough time. Things may look bleaker than ever before. There may not be any reason to rejoice in your circumstances at the moment, but you can rejoice in the Lord at any time. There is always a reason to rejoice in Jesus. His promises are eternal and He is faithful to do what He has promised (Romans 4:21). Being full of joy is a choice – you need to choose to rejoice in the Lord. You do that by choosing to focus on Jesus and what He has done for you, more than on what’s happening around you.  It’s a choice. You have to choose to rejoice. Yes, bad things happen and life isn’t always pleasant, but God is always good and we always have a reason to rejoice no matter how crazy things are.  

“O magnify the Lord with me, and let us exalt his name together.”
Psalm 34:3 KJV

Your heart will get happy when you choose to focus on the right things. When you magnify something, you make it bigger. When we focus on God and on His promises we make Him bigger than our problems. You get to choose what you focus on, and that determines whether you experience joy or not. It’s a choice to rejoice.