"Though you have not see Him, you love Him; and even though you do not see Him now, you believe in Him and are filled with an inexpressible and glorious joy." 1 Peter 1:8
So often we use the terms "happiness" and "joy" interchangeably, but they are not truly the same. Happiness and joy are actually quite different. the key point being that God promises us joy, but He never promises us happiness.
Being happy is an emotion, like sadness or jealousy or fear. It comes and goes. You might be happy about something, but you are not going to be happy all the time. Happiness is great, and we can get happiness from a lot of things, even earthly things.
Joy, on the other hand, is a state of being which can and should be long lasting. Joy is promised by God and something we should strive for. In Hebrews 12:2 it states, "For the joy set before Him He endured the cross, scorning its shame, and sat down at the right hand of the throne of God."
The constant knowledge of our salvation and God's love for us will offer us joy, even if our present circumstances are such that we aren't happy about them. There is so much joy ahead for us. In Romans 14:17, it states, "For the kingdom of God is not a matter of eating and drinking, but of righteousness, peace, and joy in the Holy Spirit."
We know there will be joy in Heaven, yet we should experience joy now in our lives. God promises joy when He promises salvation. When we keep this in the forefront of our minds, we will find peace and joy in our earthly days also.
In Psalm 16:11, the psalmist writes, "You make known to me the path of life. You will fill me with joy in Your presence, with eternal pleasures at Your right hand." God gives us joy, even when we are not happy. We may not smile all the time, and we are not promised sunshine and rainbows every day. What we are promised is salvation and eternal life, which is cause for great joy even in the midst of whatever life brings.