We, human, make our best decision when we feel relaxed. Writing down your entry and exit plan reduces tension by separating your two job analysis and trading. Analysis can be done while the market is closed, trust me when a market is open, watching the market and doing analysis at the same time can be distracting. Better to analyze in peace and quiet place and give a plenty of thoughts and time to come up with a good plan. Once you do that, the other job is to trading which is solely focused on executing your transaction.
You know why this is the effective way to make profits because Mr. Market most of the time he is crazy. Warren Buffett, the most successful investor in the world, says that when you buy a stock you become a partner of a manic-depressive fellow he calls Mr. Market. This character runs up to you every day, offering to buy you out or sell you his shares. The best thing to overcome this struggle is to ignore it, ignore Mr. Market.