The difference between an Atheist and a Christian could be summed up in 6 words spoken by Christ. Jesus began His public ministry as He preached the "Sermon on the Mount". The sermon starts out with the Beatitudes. (Matthew 5) The Beatitudes start out with Jesus saying this "Blessed are the poor in spirit".
Definition for "poor in spirit" = Those who realize they need the Lord.
Definition for "blessed" = happy.
So now our quote by Jesus could say this "happy are those who realize they need me".
Does this mean the Atheist can't be happy? Of course not, but an Atheists happiness depends on circumstances. A Christian should follow Christ's lead. Was Jesus happy?
Someone once asked Billy Graham that question, here was his reply "If by happiness we mean serenity, confidence, contentment, peace, joy and soul satisfaction then Jesus was supremely happy.
When we realize God is control our priorities will change and then our happiness wont be dependent on circumstances or things.