I think, there are a lot of reasons for it. Here you are my own list:
- lack of knowledge (learning is your key to success, no matter how experienced you are)
- untrustworthy broker (search for info about broker you want to work with, first of all)
- absence of strategy (there is no ideal strategy, but you can create the most appropriate for you)
- great influence of emotions (what you need is to control your fear and anger and stay cool)
- too risky (do not trade everything you have; it also makes you too emotional)