Two big reasons: 1) Trading psychology. Some traders ego get in the way. The friend I was teaching to trade, she had a huge position on VRX then, in a downtrend. She bought call options. I told her she was trading against the trend. I told her to buy put options to recoup her losses. You know what she did? She bought more call options, adding more and more losses! Ego got to her. She cannot believe she is wrong when the stockchart clearly shows the trend is down! Like running towards a tsunami and not away from it which is the correct decision!
2) Risk Management, no matter your account size, you must not risk more than 2% of your account in any trade. If you do not follow that, you risk blowing up and losing all your monies! So, if you have an account size of $2,000, you cannot risk more than $40 per trade. If you have an entry of $20.00 and stop loss at $18.00, total risk is $2.00 so, how many shares should you trade? $40/2 is 20 shares@$20 is $400.00 allotted to that trade.