Obedience to the law protects a Christian’s joy. Sin has no sweatness – it sucks joy out of the sinner. The law of God brings joy to those who keep it.
Sometimes the law brings momentary hardship, but overall it protects the joy of the one who keeps the law. The biggest threat to a life of happiness is a life of sin.
Man in the garden was happy. He did not sin, and did not need sin to fulfill his happiness.
The moment that man sinned is the moment he became unhappy. It is often like this that ill gotten gain brings only bitterness.
Man was made to love God. It is not surprising that since man's purpose is to be in relation to God that man will be most happy when he is in a proper relation to God.
Heaven is a place with no sin and it is were man will be perfectly happy. The reality is that sin is a hindrance rather than a help to happiness.