The market often rewards bad behavior. You exit a stock because your stop is hit. You are okay with this because you followed your plan. The market then immediately reverses. You begin to think, “If only I stayed with the position.” The next time the market goes against you, you decide you are not going to get tricked again. This time though, the market does not reverse and what started out as a small manageable loss is now huge.
The market will give you loss after loss forcing you to abandon a methodology right before it takes off without you. On the flip side, the market will lull you into a false sense of confidence. You trade larger and larger, taking on excessive risk. You print money until your risks become so excessive that one or two bad trades wipe you out.
Learn from the market, but realize that sometimes it can be a lousy instructor.