How can the Torah command us not to covet? We understand why we are forbidden to steal or murder—those are actions we can control. But jealousy is an emotion. When we see something we want, how can the Torah forbid us from desiring it?
The Ibn Ezra offers a profound insight: people naturally desire things they see as attainable. A person is unlikely to covet something completely beyond his world or imagination. The Torah is therefore teaching us to train ourselves to view what belongs to someone else as outside the realm of what is meant for us.
A storekeeper once complained to Rabbi Meir of Premishlan that a competitor had opened a store nearby and was taking away his customers. The rabbi compared him to a horse that sees its reflection in water and mistakenly thinks another horse is competing for the same water.
There is enough water in the river for everyone. Likewise, what G-d has prepared for someone else does not diminish what He has prepared for you.
Instead of comparing ourselves to others, we should focus on appreciating what we have and trusting that our own path has been prepared for us.