It is the desire to have/accomplish more than others. Jealousy will rise up when others are better than you, so you'll want to be better again so you'll do what it takes. It's greed, power and money what keeps us going. The more you have of power, greed. The lesser you'll think of others. The lesser you have of money, the greater your desire to get money.
tl;dr:
we always want more.