The reason for stopping at a daily target is to protect your gains and avoid giving them back. Generally speaking, you should have a goal as to how much you would like to earn on a yearly basis, and base your daily goals on this. I find if you don't have a goal, and you just want to make as much money as possible, you find yourself getting greedy and you end up giving back all of your profits. What I like to do on days where I hit my target is paper trade for the rest of the day, for practise. More often than not, I'm better off not having trading real money for the rest of the day.
Also, once you're up on a day, you tend to get a little overconfident and feel like you can risk what you're up. What usually hapens in this case is you end up giving your profits back and then some.
