This is really something which everyone should understand.The reasons for losses can be numerous and all of them are quite individual.
Some traders simply don't have sufficient amount of knowledge for this activity. There are thousands of people who think that everyone can come to trading and start making profits immediately. These people are lulled by the very image of trading and they tend to think that all the traders do is just clicking buy/sell buttons and nothing more. However, it is impossible to show the actual process of making decisions of a professional trader. They do lots of hard work which stands behind the process of opening the deals.
Other traders do have sufficient knowledge but they are affected by the emotional factor which distracts them from common sense and logic. Psychology plays huge role in trading and only skillful and emotionaly stable traders can make decent consistent profits.
Some experienced traders may also have losses just because of the fact that they don't update and refresh their knowledge and trading strategies. It is an open secret that market is developing all the time and all kinds of knowledge become outdated quite rapidly. That is why it is vital for a trader to be on the move all the time and constantly modify their trading strategies.
These reasons are the most common, from my perspective, but there can also be some details which are individual, so if you want to see the reasons of your losses, you should pay attention to your trading strategies and the way you take your decisions. The best way to track down all of your mistakes is to make a trading diary where you should write all the thoughts prior to the deals.