Christian joy is more than happiness, which is an emotion. Joy is more accurately defined as an inner state of peace and sense of well being regardless of outward circumstances. Another key ingredient to a joyful life is having a thankful heart. And, a thankful heart is the direct result of remembering and reflecting on who God is and what He is doing in your life.
We lose our joy because we fail to honor God and give Him thanks (Ro. 1:21). On the other hand, the Lord tells us through the psalmist, 'He who offers a sacrifice of thanksgiving honors Me' (Ps. 50:23) Our joy is primarily contingent upon two things: 1) Our understanding of God; and, 2) The state of our communion with Him. If joy is the enjoyment of God's presence then sin kills our joy.
Ingratitude is sin. If gratitude is expressed from the heart by joy, praise and exaltation of God, then ingratitude is expressed from the heart by grumbling, discontent and complaining. These are not minor sins as the Israelites learned in the wilderness (see Num. 14).
Joy flows from a heart that is thankful. 'For what thanks can we render to God for you in return for all the joy with which we rejoice before our God on your account' (1 Thes. 3:9). Paul was constantly expressing thanks for God's grace in the lives of believers; their faith in Him and their love for each other; praying for them 'always.' If we would know the joyful Christian life, we would do the same.