Q: What is forbidden in the tenth commandment?
A: The tenth commandment forbids all discontentment with our own estate, envying or grieving at the good of our neighbor, and all inordinate motions and affections to anything that is his.
James 3:14–16 But if you have bitter jealousy and selfish ambition in your hearts, do not boast and be false to the truth. This is not the wisdom that comes down from above, but is earthly, unspiritual, demonic. For where jealousy and selfish ambition exist, there will be disorder and every vile practice.
I think it's important to note this commandment doesn't just forbid dissatisfaction with what we've been given, but also envying and grieving at the good of our neighbor. To me, "grieving" is a strange word to use here. We usually think of grief as a proper response to great loss and sadness in our lives. We grieve when loved ones die, or when someone falls into deep, unrepentant sin, or when we or someone we know and love suffers some kind of tragedy. These are all right expressions of grief. But as the catechism points out, there's another kind of grief, a sinful kind of grief that actually flows out of our failure to keep the tenth commandment. We can actually be overtaken, overwhelmed with sadness and grief because we covet what our neighbor has. For anyone who has experienced the "right" kind of grief, that seems like a remarkable statement. It's hard to imagine that envy could lead us to feel the kind of deep and encompassing sadness we have felt when grieving in the "right way".
Does the catechism go too far in using the word, "grieving"? Perhaps not. There are many cases throughout the Bible where envy has actually led someone to commit a violent act against another person (Ahab in 1 Kings 21, for example). We can look at our own world and see tremendous acts of violence committed against each other because of envy. Perhaps this is why Shakespeare referred to jealousy as "the green eyed monster". As the people of God, it's a monster we have to constantly fight and guard against, or else we risk finding ourselves being taken down a road that is truly grievous to ourselves, our loved ones, our neighbors, and to God himself.