Now the LORD saw that Leah was unloved, and He opened her womb, but Rachel was barren. (Gen. 29:31)Of course this drove Rachel crazy with jealousy so she "gave" her husband her handmaiden to sleep with. (Apparently surrogacy is not a modern concept.) After the maid gave Jacob two more sons, Rachel said, “I have had a great struggle with my sister, and I have won." There are many wonderful reasons to have children, but competing with your sister is not one of them. Although the babies were technically Rachel's, there was still a stigma attached to not being able to bear her own children and she longed for a son. When she finally gets pregnant after her husband has 6 sons from Leah, 2 from Rachel's maid Bilhah, and 2 from Leah's maid Zilpah (but who's counting), Rachel exclaims, "God has taken away my disgrace."
Her long wait is over. She has a son! Her heart's desire has been granted. The thing that she most wanted in life is hers. She should be happy right? The child she prayed for is in her arms. She should be content right?
Another example of someone who let what he didn't have steal the joy of what he did have is Haman. If you read the book of Esther, you'll know that Haman is a bad guy. For some reason the King elevated him above all the nobles, gave him a seat of honor and commanded all the officials to bow to him. Haman was probably very smart and very rich (or perhaps just very manipulative.) He was also very prideful. There was one man who refused to bow down to him and that drove him crazy. It made him so angry that he didn't want to kill just the one man (who was a Jew), but he wanted to kill the entire Jewish race. He got the King to endorse his extermination plan, not knowing that the Queen was Jewish.
Haman had everything he could ever want. "
Haman's discontent led to his downfall and his death when he was hung on the gallows he built for Mordecai.
Discontent probably won't kill you like Rachel and Haman, but you will be miserable. It is possible to live a long discontent life... but why?