Mark Douglas:

"Regardless of what you may have planned
or wanted to do, any number of psychological factors can come into
play, causing you to become distracted, change your mind, become
scared or overconfident: in other words, causing you to behave in
ways that are erratic and unintended.
Because gambling games have a formal ending, they force the
participant to be an active loser. If you're on a losing streak, you can't
keep on losing without making a conscious decision to do so. The end
of each game causes the beginning of a new game, and you have to
actively subject more of your assets to further risk by reaching into
your wallet or pushing some chips to the center of the table.
Trading has no formal ending. The market will not take you out
of a trade. Unless you have the appropriate mental structure to end a
trade in a manner that is always in your best interest, you can become
a passive loser. This means that, once you're in a losing trade, you
don't have to do anything to keep on losing. You don't even have to
watch. You can just ignore the situation, and the market will take
everything you own—and more."