Let's say it was the main reason why many people are losing but also this is a reason why people reach their goal because of their aggressive pursuit and greediness to reach their dream. Therefore, I'd say that greediness is bad if mismanage but if it is doing good and controlled, that absolutely a reason for our success. It fails because we fail to decide the right thing and many people got this.
Reasons people are losing are very common. We all know that because it is taught in many courses, groups, communities but what is taught is on paper, in theory. To have loss or profit, people must actually participate in market, with real money, not with demo trading.
When it comes to something real financially, emotion and ability to control emotion decide loss and profit. Very rare people can control emotion and become winners in the market. Others can not control their emotion then they make emotional decisions that lead to loss.
When greed takes over, the investor becomes a gambler and a gambler always loses against the house. Been there, done that.
An investor should never ever let his emotions affect himself. That's the most dangerous thing an investor can do to himself, losing his control over his emotions.
"Be greed when others are fearful and be fearful when others are greed". This saying is very famous but to apply it successfully, very rare people can. The market is zero sum and a minority of winners own most of capital and profit in the market.
A good or best way to eliminate emotion is to invest long term. Then you will not care about emotion and reaction with market daily or weekly.
