Many people of God are simply not happy.
Their happiness is based on externals, stuff like personal academics/self achievement, relationship or finances: on how much they have attained over the weeks/months in sales figure, how many numerical they have in their bank accounts, how much property in their possession or acqusition, how their relationship with their spouse/ kids fare, how a certain person or group of people have treated/behave towards them. While all these has a place of its own, it's sad to watch how the people of God loses their inner joy over all these.
I've met another group of unhappy people who live their lives centred solely on a set of laws and commandments that's derived from their sacred book. They can remember so well the text inside out, many who can quote verses so meticulously, many who can "proof-text" their doctrine.
But if you're not happy, you're just living a rigourous/tiring religious life of do and don't and that sucks. A wise man said something to this effect, " A person can be in the known religiously but spiritually undiscerned." Religion and spirituality are two differently things.
The Good News (Gospel) is about how one can meet God, the Author of the sacred book, enjoy God being with us and live life happily, much less to do with other stuff.... And unless you discover the God of the sacred, you're just living life on the peripheral (by the book).
You must be happy because of the simplistic fact that
1). Immanuel (God is with us). Was once seperated from God but now reconciled.
2). Identity changed to being child of God. To those who believe, God gave you the rights to become a child of God.
3). God gives you a Holy Spirit to empower you and gives you the ability to live a life as a happy child of God. In the words of a child, "spirits and demons jumps out and Holy Spirit jumps in."