"And we know that God causes all things to work together for good to those who love God, to those who are called according to His purpose. For those whom He foreknew, He also predestined to become conformed to the image of His Son, so that He would be the firstborn among many brethren..."
Joy - as separate from happiness - can be present even in the midst of great sorrow. Joy is the peace and thankfulness born of faith that God will always work all things for your good and His glory. Happiness can come and go, but joy remains. It is born of the fact that Jesus is Lord, and I am saved. (See more on that in Weekly Impressions - Big Picture Check.)
I am not saying you won’t experience happiness in your walk with God. Joy leads quite frequently to happiness (though joy can be difficult to hold on to in many circumstances). I only mean to stress that happiness is not the goal of a relationship with Jesus Christ. The goal is the relationship itself, born of the joy of His love for us, shown most extravagantly through Jesus’ sacrifice for our salvation.
I have previously fallen into the trap of only pursuing happiness in my walk with God. It changed my prayer life and the amount of time I spent with Him. My prayers became my list of desires, and I spent less and less time with Him. In fact, it contributed to bitterness. Once I was able to take my personal happiness off the throne where God belonged, my life became so much richer, because my relationship with God was so much richer.
When I find myself asking, “Why can’t I have this?” or “Why didn’t God do that?” I have to ask myself why I’m asking those questions. Is it to learn more about Him and how to follow Him, or have I let my desires take God’s throne? Have I let my own desires rob me of my joy?