The last of the Aseres HaDibros is, "Do not desire your friend's wife, his slave, maidservant, ox, donkey, or anything that belongs to your friend" (Shemos 20, 14). The Ibn Ezra asks a question that bothers many. How can the Torah command us with regard to an emotion? How is it possible to prevent one from desiring in his heart a beautiful person or object? He answers with a parable. An astute peasant who catches a glimpse of a beautiful princess will not desire her hand in marriage, because he knows that there is no possibility in the world for such a marriage to take place. Similarly, every intelligent person must acknowledge that a beautiful wife or desirable possessions are not granted to a person because of his brains or brawn. Rather, they are allocated by Hashem to whomever he chooses. Hence, one who understands that Hashem did not wish to give him that particular object will automatically refrain from desiring another's possessions. Moreover, he is aware that no amount of scheming will succeed in his obtaining the sought after item - it is out of reach to even a greater degree than the marriage of the princess to the peasant. He should place his faith in Hashem that He will provide him with sustenance.
Therefore, one who has true emunah cannot possibly covet another's possessions. Rav Wolbe (Shiurei Chumash) comments that many people have difficulty with such an explanation, because their emunah and bitachon end when it comes to the practical application. Some people claim to have bitachon in Hashem, but if a competitor opens a business across the street from them, they are on the verge of having a heart attack. "But don't you have bitachon?" a friend might ask. To which they will retort, "Of course I have bitachon - but at the end of the day he is going to take away my livelihood!" Their bitachon lasts until it is put to a real life test.
The Ibn Ezra understood that emunah is not a matter of theory. It must be tested to be proven true. Yaakov Avinu lived with the belief that everything that he had was allotted to him by Hashem, and therefore it was incumbent upon him to look after his possessions to the greatest extent possible. When it came to a real life test, he extended himself to claim some forgotten vessels; if Hashem gave it to him, then it was for a purpose. Likewise, living with the realization that whatever Hashem gave me is mine, and what He did not give me cannot possibly be mine, precludes one from feeling any desire for the possessions of his friends and neighbors.
It might do us well to spend another few minutes on the Ramban at the end of Parshas Bo. It will help us inculcate the knowledge that Hashem is the Creator, Knows what is happening, is involved and Omnipotent. One who has absorbed these concepts will be able to fulfill the Tenth Commandment with greater ease.