I think both are possible reasons to make the gambler lose control but thats is not always the case. Not everyone who wins big or losses big, losses control. Some choose to stay in control of the game and just enjoy it. Let us not hope for the worst.
Anyway, you see when some wins big, it creates overconfidence to win consistently that makes gambler hope that it can happen once again, that it is their time with luck. Then a significant loss can be emotionally distressing. The desire to regain what was lost then disappointment can drive compulsive behavior. Gamblers may think they can recover their losses and return to the previous state of success. Thats why I always think that regardless if it is a big win or loss, gamblers should learn to pause. Its our mind and emotions that pushes us to decide impulsively.
You've made a remarkable point, about staying in control. While gambling, the gambler holds full culpability on their decision. Whether to lose control or not. The demoralizing factor is their emotions on the results of other gamblers. Failing to acknowledge the individual difference in gambling experience, leads gamblers to choose a wrong route in decision making. A player that possesses critical thinking, has better advantage over others who don't. Continuously, playing games due to the outcome of another gambler's bet, restructures the self control of the gambler to think he deserves to win, like everyone else. If he is in control, what matters to others won't be meaningful to him. Hence, he'll move ahead with his conclusion of hitting the exit button. Notwithstanding the success of the player next to him. Healthy thoughts, facilitates in minimizing loss of control as gamblers. As the brain would keep reminding us of the repercussion of trying again; running broke. Which brings forth anxiety. When we win, it's superb and worth celebrating with more trial, to see if it's our most lucky day. But, the opposite may surface as a result. Then, our thoughts should also go to the opposite side. Gambling is a game of mind. Refusing to respect our instinct, is wrong, mainly in gambling. The possibility of experiencing a simultaneous win, isn't one in gambling. When a player wins twice or thrice in a roll, it's a trap. Though we call it luck. But, when the losses starts appearing, withdrawing from the initial happiness easily is very hard. The player wants to hit another joy again and again. Here, he'd lose all his wins, trying to regain the money he lost. What stopped him from stopping, needs to be studied.