This is a verse that we throw around a lot as Christian's but what do we mean when we say it?
The verse actually comes from Nehemiah 8 verse 10 so has a lot of context that we throw away by only using this part of the verse. The people of God have just re-built the wall to the city and have heard the law of the Lord read to them and they are worshiping God.
God's people were weeping because they have heard God's word read and realise how short of God's standard they fall. Nehemiah says go and celebrate and share with those who have no food to celebrate.
The end of verse 10 says; "This day is holy to our Lord. Do not grieve, for the joy of the Lord is your strength." The people have renewed their relationship with the Lord and the joy is in the fact that God continues to have a relationship with them.
Even though they are an adulterous people, the Lord continues to draw them back into his loving arms. His joy is in His people. Us.
So my strength comes from the Lord's joy in me. God is overwhelming infutatuaed with us as people and will do anything to keep us with him. Don't forget, he sent his son for you.
Your strength comes from knowing God delights in you!