Jealousy is a very bad character-trait, as the Tanna says in the tractate of Avos, “Jealousy, desires and honor take a person out of this world!”. Behold, the inclination of the heart of a person is evil and desires always to be unique in his generation in wisdom, good deeds, wealth and honor, and because of this he is upset if there is another person who has the same as him or is greater and better than him, and he will be jealous of him and seek evil for him and tell bad things about him. Hatred which comes about because of jealousy is a very great hatred which is like fire which devours with no cure. One who has this terrible character-trait, will find all of his days will be sad and painful, he will eat the bread of sadness, he will have no close friends, he will encourage many quarrels, will be happy when there is trouble and will want his friend to do badly and that his friends should stumble in halachic matters and should minimize their service of Hashem, and many other terrible things and sins which come from jealousy.
Who can tell the greatness of the evil of jealousy! Whoever is a man who wishes life will flee from it and be like a warrior who conquers and overcomes his evil inclination through thinking profound, holy, pure thoughts, whereby he believes with a complete and pure trust that no person can even touch that which was prepared for his friend, and even if he would be alone in the world he would be unable to gain more money than that which it has been decreed that he will receive. In the same way he must believe that even if many other people (even tens of thousands) will join his own line of business, he will not lack from that which it has been decreed that he will receive. He should want only that which is the will of his Creator, who is the Rock all of whose works are perfect, and know that the Creator gives to each of us that which is good for him.