It's actually funny for gamblers who have won and then try again to get another win. Gambling is indeed to get a win, so when you get it, why not enjoy it?
Dissatisfaction and greed may be factors, but I'm sure gamblers who do it must be addicted. Because the victory obtained has not satisfied their desire to gamble. Even though they return to gamble, at least they have to feel the victory and leave a little to continue.
It's funny, for sure, but I believe it's more because of greed than addiction, or maybe a mix of both, because when you are not addicted, you may control your greed and don't do it, but if you are both greedy and addicted, there is no way you are going to stop from giving it another try only to see if you can get another big win or not because you've already got one, so maybe you can get another. Most gamblers lose what they had won along with their initial capital only because of this mistake, and then they regret it. Isn't that even funnier?
Human beings are totally incomprehensible sometimes. As you said, if someone is gambling to win something good, they should be happy and enjoy if they have got it already, but as soon as they get it, they start wishing for more out of greed. This isn't only about gambling, but this happens in everything in general life. When you wish to have wealth, you don't stop desiring for more once you get it, but you still think and wish that you had more than how much you currently have.
We humans are so ungrateful sometimes.