The biggest problem IMO is being able to stomach trading a very large fraction (or more likely, multiples) of your annual salary and not being affected by very large red numbers. If you stop thinking rationally, you lose. If you experience too much stress, might as well sell your time instead and avoid the drug habits.

In other words, you have to be able to put your house on red, and restrain yourself from doing it unless you're certain the numbers are in your favour.

I don't think many people can do this.