Maybe it's because everyone wants to be effective and once you reach your goal, maybe you lose effectiveness. Or maybe it's like when you're at work and you get a project done early and then you usually piss around for a bit because you're done your work anyway, so you can do what you want now. Or it could be a probability thing, as in the more you trade the more chances there are that something could go wrong. Or maybe everyone has a subconscious limit of money that they believe they should earn each year and they get uncomfortable if they think they're going past that, so they subconsciously sabotage themselves to match their perceived income limit that they think they deserve. Who knows.