And you shall rejoice with all the goodness that your God has given you and your household.
This commandment applies to those who have harvested and gathered their harvest. Why did God have to command them to be happy? Wouldn't a farmer who had a great crop automatically be happy?
Here we find the perverseness of human nature which never allows a person to be content with what he or she has. Even when a person succeeds he or she remains restless, desires more and more again and worse measures his or her harvest and attainments against those of his neighbors. If they have more then jealousy consumes him and his Joy is marred. This jealousy robs a person of his or her peace of mind, happiness and contentment and generates bitterness, greed and hatred.
The Torah is telling us that we have to be happy, and content with what we have. And this is the famous rule in Ethics of the Fathers is it 'who is a rich person? A person who is happy with what he or she has.'