They focus more on winning again and they become greedy thinking that they could easily regain the losses which usually ends up in gambling addiction. They feel confident that they could always win because they have experienced it already which is a wrong mindset.
Greediness is one of the reasons, indeed, but not the only one. Gamblers tend to forget about their previous losses and how much they have spent every time they hit a big win. They consider their winnings as free money which is a wrong belief as they could have spent more than what they earned (not talking about those who made insane wins). Because of this, they usually wouldn't mind spending more and even start placing higher bet. And we all know how this usually ends!