Excitement is what makes someone forget that he has to secure his profit upon winning those bets. And also being confident that he'll never lose on that day is what stops him from doing it too. Probably we're mentally strong when we're able to cash out and take the profit but I wouldn't say that those that didn't are mentally weak. But it's true that they need to control themselves and forget not that they're gambling for the money and just having fun is just next to it. No matter how hard we try to say that we're gambling for fun, when the profits are there you shouldn't neglect it and don't ruin the fun by losing more and forgetting that you need to take and cash out for a while before continuing.