I see trading as more of an emotional game, so if you fail to control your emotions, you are bound for losses because it will make you make rash decision that you wouldn't have made in a normal day.
Another thing why emotional control is very much important in trading is that you might be very skilled in the craft, but if you fail to control your emotions, all that your skills are practically useless because emotions will make you lose faith in your analysis and you will be pulling out of a trade you should have won if you stayed longer, and staying more in a trade you would have cut your loss earlier, so emotional control in trading is one of the major skill a trader must have in other to be successful in the craft, and the best way to do that is by trading with an amount you are willing to lose, so that if things goes south you wouldn't even give a shit about the losses, but if you trade with an amount you cannot lose, just know that you are actually putting your emotions on a rollercoaster ride which you can't comes out from successful from, that's is what I can bet on.