mostly bcos they're insecure of themselves. there are some types of jealousy and i'm not saying thats its all bad. like, jealousy of loving someone, its good as long as its not exaggerating.
I do not agree with opinion of jealousy which stems from love is not dangerous. According to me, All kind of jealousy are completely toxic for mankind. If a human being desire to become a glad person in life, he or she must stop envy people around them.