How is it, people often ask, that so many professing believers have so little happiness in their religion? How is it that so many know little of joy and peace in believing, and go mourning and heavy-hearted towards heaven?
The answer to these questions is a sorrowful one, but it must be given. Few believers attend as strictly as they should to Christ’s practical sayings and words. There is far too much loose and careless obedience to Christ’s commandments. There is far too much forgetfulness, that while good works cannot justify us, they are not to be despised. Let these things sink down into our hearts. If we want to be eminently happy, we must strive to be eminently holy. (Taken from Expository Thoughts on the Gospels, John 14:21-26).
This doesn’t mean that all unhappiness in the Christian’s life is because of some lack of obedience, but I do think it accounts for much more of our unhappiness then we often realize.
For some more thoughts on Christian happiness, start here.