To fail to rejoice is unchristian. It would be better not to have been born than to not rejoice.
Why is joy so important?
Imagine that I gave my wife $500.00 and asked her to spend it all on herself--from head to foot. She then takes that money and buys me a set of books. That sounds virtuous; but is it? I like books. She may have even bought me the very books that I wanted. But, there was something else that I wanted more. I wanted her dressed up. Knowing that she wanted new things--I gave her the money and expected that she would spend it all on herself. I gave her the money with a sense of happiness that she would use it as I asked and that she would show up--dressed up.
Her gift to me would be to receive my gift to her and to delight in it and therefore in me. I did not give her money to buy me something for my independent use. My vision was that the money would bring both delights to her and to me. She would be delighted because she spent as freely as $500 allowed on things she wanted and because I asked. I would be delighted in seeing her delighted and in seeing her in a new outfit. She honored me by using the money in a delightful way.
God gives good gifts to his people. He gives food, drink, work and family. And for his people he also gives a capacity to enjoy his good gifts. What if we take his gifts and do not use them for his glory? We might imagine that the best Christian life is one that is somber and sad; that somehow if a smile graces our face that it disgraces the cause of Christ and makes us a lover of this world.
Loving this world has more to do with trying to suck out the pleasures that this world offers in a way that is independent from God. Rejoicing in God is to take all of his good gifts and enjoy them with gusto. They have come from God who loves us. They are not to be squandered in riotous living but neither are they to be hidden beneath black draping.
Sackcloth and ashes are not to be worn by those who rejoice too much--but by those who rejoice too little and in the wrong way.
We give to God by enjoying his gifts. Perhaps it is a great mystery but somehow our joy in God brings joy to him. Our using his gifts with great delight brings a smile to his face. We do not glorify God by receiving our food and then saying, Lord, I can't take this--I give it back to you, and then walking away from the table. We glorify God by eating the food, letting the taste buds dance, and saying, thank you God for your kindness--that was good!
See Ecclesiastes and Philippians for a defense of my position on joy.