The Ten Commandments are repeated in this week's Parsha, ואתחנן. There are several differences between the text in פרשת יתרו and in ואתחנן. One such difference concerns the final commandment.
In יתרו, the last commandment ends with the words, לא תחמוד, which we translate to mean, "Thou shall not covet." But in ואתחנן it says, לא תתאוה, which has more of a connotation of lusting after that which our neighbor possesses.
The מכילתא explains this difference in words to teach that לא תתאוה is the first stage of coveting, and it leads to לא תחמוד, where one acts on this high level jealousy.
The לא תחמוד represents obsessing over the other person and actively wishing bad on him. This is also described as a serious רוח רעה, bad spirit that overtakes a person. It could even lead to committing crimes against that person, such as stealing or slandering.
Of all the negative personality traits that one can have, it is probably jealousy. It takes away the possibility of being happy with one's lot and prevents a person from being happy, in general.
We must work on our Midot and remove all jealousy that we possess and learn to truly be happy for other people's success. Shabbat Shalom