Many people lose in trading and majority of the people that loses in trading are the people that are seeing trading as a gamble. If you see trading as gambling, you will keep loosing money. But if you see trading as a business and study it fine, you will gain.
Even in gambling, an experienced gambler will make more profit than an inexperienced gambler who only depends on luck to survive.

What you call greed can be summarised as risk management and it is a great factor in trading.