Look at it this way...
Put... like $100,000 in a mutual fund. Get a 15% return every year. So you make, in simple terms, $15,000...
Now let's say you get a deal from the mutual fund that tells you what they buy and want you to execute the trade. Same performance, you make $15,000...
Now, let's say you develop a system that consistantly returns 15%. You execute the system and fortunately the system hasn't faded and you make $15,000...
OK... now you are a discretionary trader. You trade hear and there, depending on the market. (Well, considering you've got rules and stuff, not random ) You trade for a year and end up making $15,000.
Next roll a dice and Buy or Sell if the roll was even or odd. Eventually make $15,000 at the end of the year.
Completely, cluelessly, and randomly pick trades. You make $15,000 at the end.
....
Now, any one get the point of all this?
Hint: Instinct. Not only human but all animals.
PS. People should stop reading trading psychology books...