Those who have jealousy, suffer. The Reishis Chachmah writes, "Someone who has kinah (jealousy) is 'robbing' himself. He is always sad. Everyone finds sweetness in life, except for the person who suffers from jealousy. He cannot enjoy his food if someone else has something good."
The Rosh (Orchos Chaim) writes, "Jealousy is a disease that doesn’t have a cure."
The Mesilas Yesharim writes, "There are people who are so foolish, that when they see their fellow man has something good, they become very distressed and upset about it. Because of their sorrow, they don't enjoy anything that they do have... There are others who aren't so upset, but they also have some amount of remorse. They aren't calm when they see someone reach a level that is above them... Most people are at this level..."
Any degree of jealousy destroys one's level of enjoyment in life, as it says (Mishlei 14:30) קנאה עצמות ורקב, "Envy brings rotting of the bones." If we could free ourselves from this illness, we would lead much happier and more satisfying lives.
Think about the people you are angry at, the people about whom you are tempted to speak lashon hara. And now ask yourself, "Perhaps you are jealous of them?" Is your anger rooted in jealousy, because you think they have what should be yours?
The Rebbe of Kotzk zy"a said: Most bad middos begin later in life, but jealousy begins when a child is born. One-year-old children get upset when someone gets something they don't have.
Once, chassidim of various courts were speaking together, each one was discussing an aspect of the greatness of his Rebbe. Rebbe Moshe Minder zy"a, from the esteemed Slonimer chassidim, was also present. Someone asked him, "Reb Moshe? Why are you silent? Tell us something about your Rebbe." Reb Moshe Minder replied, "All I can tell you is that the three middos, kinah, taavah, and kavod, which the entire world chases after, my Rebbe runs away from them like from fire." If we understood how much we lose from kinah, how it eats away from our enjoyment of life, we would also run away from it as we run away from all matters of life that harm us.
The Agra D’Kalah (from the Bnei Yissaschar) zy"a explains that the Aseres HaDibros are a synopsis of the entire Torah, and the tenth of the Ten Commandments (which implies the culmination and the most important of the Aseres HaDibros) is תחמוד לא, the prohibition of jealousy. This prohibition, therefore, is the heart of the entire Torah. He explains that it is the primary point of the Torah because the main lesson of the Torah is emunah, and overcoming jealousy means believing that each person receives the portion that is bashert for him. With this emunah, he won't be jealous of others.
The Orchos Tzaddikim shows how one can transgress all of the Aseres HaDibros due to jealousy. Because due to his jealousy, he will break into his neighbor's home to steal on Shabbos, violating the Shabbos and the aveirah of תגנוב לא. When he is rebuked for his theft, he will kill, transgressing תרצח לא. His parents will rebuke him and he will hit them. He will swear that he didn't steal, and thus, transgress תשא לא. When he is caught with his aveiros, he will leave the Jewish people because of his embarrassment and become a goy who worships avodah zarah. So he can transgress all of the Aseres HaDibros (and the entire Torah) all because of תחמוד לא, jealousy.
