The joy of the Lord is our strenght. It should not be confused with happiness. I have experienced His overpowering joy even when going thru some of my worst trials. Joy is independant of circumstance. Form deep within our spirit this flows until it fills our lives in an overpowering way until it flavors all we do. Are there days when I do not have this joy? Yes. But generally they are days when my focus gets on life and it's problems. Even now as I think of my love for the Lord I feel His joy welling up in me.
1Th 1:5 For our gospel did not come to you in word only, but also in power and in the Holy Spirit, and in much assurance, as you know what kind of men we were among you for your sake.
1Th 1:6 And you became imitators of us and of the Lord, welcoming the Word in much affliction, with joy of the Holy Spirit,