Jealousy can ruin a life, for example; say your dating someone who you love more than anything and anyone, then you discover one of their friends has a massively huge crush on them. Now, your first reaction would be, "oh I see..." then you quietly think to yourself, "If he tries anything I will personally see to his end." This is a sign of jealousy mixed with the will to protect your girlfriend.
Now, your jealousy continues for awhile then you become blinded by it. You make rash decisions and some of those could end your life, or cause the separation of you and your loved one.
By the time it's all over, you realize (all to late) that you have just lost the love of your life and caused yourself much more damage, emotionally and physically, their best friend hates you even more, and you hate yourself. This is only one of many different scenarios where jealousy can ruin your life. Don't let it cloud your judgment and destroy your all to perfect life.