Partly because of the rubbish price action. You think it's going up and enter long with a stop - price goes down and hits your stop and then goes up. At this point you may think there is an uptrend and you re-enter long with a wider stop - trying to let the trade 'breathe'. After some time you realize you went long at the very top.
Now you think - I shouldn't give up, let me try short. You enter short and price goes your way. You've read somewhere it's good to 'run the profit' so you stay and try not to be scared by retracements. Price goes up and up and now your small profit turns into break even, then small loss... You're still trying to keep calm as you've heard somewhere about new traders being nervous and jumping in and out too often. At this point price goes ballistic and hits you stop and on top you get slippage.
After this you realize it's a sideways market, so you'd better start using a different system.
Next day you try to catch the tops and bottoms, but then after few losses you see it's a trending market.
Then there are some important speeches, reports etc. The price jumps up, drops down and at the end comes to pre report level. If you are a trader who likes using stops all the time, your stop is hit at this point usually with slippage.
These are only few things I can think of. I've never mentioned unscrupulous brokers , vendors etc.