He only got moved by the fact that he can't lose to a novice, as you said. That's the fall of most men today, believing that some people can't be better than them in their field. Gambling is not a game where we'd be moved by the reactions of people around us, like in the story, the man was bothered by the laughter of those around him. He wasn't prepared to face the shame and, hence preferred to lose more money to cover for it, thereby chasing loss. The story is nice and also helps to prove that some professional gamblers can still get addicted to gambling. Because at some point, they won't be able to control their emotions and make a series of mistakes, without knowing or being aware of it. We all have different motives and our actions change each day of the gambling session. The game can change, and the results may seem promising, so the gambler may play reluctantly, trusting that he will win, just in a few predictions.
A professional can be challenged with such a situation, where he thinks it's his day and nothing can stop him from seizing the opportunity. Not realizing that no gambler has a day. Outcomes or results change, hence nobody is entitled to winning, alone. A newbie can win more than a pro, someday. Those who gamble because others did, will deceived into thinking that if a newbie wins, while won't there win also. In the process, they'll end up losing all their bank roll and rush to take loans to gamble, like in the case of the man in the movie, illustrated above. Once it gets to taking a loan the gambler is close to becoming addicted, as he can't wager games with his full mind anymore. He will play to gain back enough money to fund or pay back his debts. Therefore, putting himself in a more severe problem, all because of a mistake he made out of ego or trying to prove that he's a competent gambler. Anybody can get addicted, not minding his level of gambling knowledge, pro or new.
If someone feels arrogant and boasts about himself and feels better than others, he will face defeat from others. Sooner or later, this will happen, and as a professional gambler, he should be able to control himself and choose not to continue gambling instead of borrowing money and losing all his money. And yes, even a professional gambler can become addicted to gambling if he loses control of himself so much that he doesn't realize that he has changed just because a beginner beat him. It would be really sad if a professional gambler experienced that. That means he still needs to learn many things so that he is not affected by anything that happens while he is gambling and can remain a wise professional gambler. A professional gambler should be able to act better than a beginner and can take care of himself well too.
But usually, a professional gambler who is challenged by a novice gambler will underestimate the novice gambler's abilities so that he seems to be just gambling carelessly. But after a few rounds, he must have seen that the novice gambler had better gambling skills, but he didn't want to admit it. It turned out that after gambling for a while, the professional gambler was defeated by a novice gambler, and he couldn't accept it. If he ends up being provoked by his emotions and wants to avenge his defeat, he will not stop himself but will instead be even more eager to beat the novice gambler. Conversely, the novice gambler can actually carry himself calmly and not be provoked by the professional gambler and ends up winning the gambling game. It was a sad ending experienced by a professional gambler, and it would be a matter of pride for the novice gambler because he managed to beat a professional gambler.
Reading your last paragraph, I was able to determine the reason why the novice won the game. He wasn't anxious about winning. He played the calculative game and was not bothered if he won the round or not. Hence his ideas became accurate and calculated, due to the fact he was not rushing to win or prove a point to anybody. He also didn't care about what people would say, after all, he's a novice and the people are around cheering him up for his previous wins. His further loss won't matter. He never panicked about the outcome of the result. Compared to the professional gambler who wasn't at rest. He only needed to show his friends that the novice player couldn't win and he may have bragged the first time, that he wasn't serious and only allowed the novice to win the round. While he was trying to balance the game and the thoughts of the people watching the game, his mind was now compounded with different thoughts. He wasn't focusing on the game anymore.
Instead, what people are saying about him clamored his decisions and he made lots of mistakes which he won't give account for, even after losing the game. He's now on the verge of chasing losses and borrowing money just to win the novice. These two people have passed a message to gamblers. The winner, a novice gambler, didn't stress himself on the win. He even had in mind that he'd win the game. Playing for the fun of being with a professional was his first joy, and the win didn't bother him anymore. When one is less concerned about winning in gambling, he would make good decisions that can make him win from the predictions he's placed. While those chasing after the win may not win, and when they win, the money still goes back to the house. Because they don't get satisfied. The professional wasn't satisfied with the losses he had accumulated, that was why he went for a loan. Until he now came to his senses to stop and receive the shame of losing out to a newbie. I don't understand why some pro gambler would underestimate a newbie, without acknowledging the fact that gambling knows no newbie or a pro.