How and why is Lily' s behavior deemed impulsive in The House of Mirth?
Lili Bart's behavior defines her as a stubborn, helpless and superficial woman. Yet, she unveils and underlying sense of character and dignity through vicissitude that separates her greatly from the lot of opulent and narcissistic people with whom she associates.
This being said, the impulsive aspect of Lily's behavior has little to do with Lily's nature, and more to do with her battle with society: she can either swim or sink. Hence, the choices that she makes are a clear indication of desperation on her part as she sees her time, and opportunities, running out as fast as money seems to run out of her hands. As she matures, she slowly begins to recognize her real position within society: that of a mere entertainer of the rich. This realization makes her want to do whatever it takes to catch up with them and be more than an appendix of their society, and become a full part of it. Here is where her desperation kicks in and the tragic flaw of her character comes out.
...she had taken her daily meed of pleasure without caring who provided it. Now she was beginning to chafe at the obligations it imposed, to feel herself a mere pensioner on the splendour which had once seemed to belong to her.
In chapter 4 we find one example of Lily's desperate attempts to keep up. During her visit to Bellomont, Lily had to sit at the gambling table to be a good sport and keep up with the other visitors. However, her mode of thinking was very different going into the game and, to her shock, she could not control the impulse of the moment. As a result, she almost lost everything she had brought with her as far as money. It is the shock of it what shows that Lily acted on impulse, and out of the ongoing need that defines her character: that need of being "one of them", to the point of putting herself in a very vulnerable situation.
she could not conjure back the vanished three hundred dollars. It was the sum she had set aside to pacify her dress-maker--unless she should decide to use it as a sop to the jeweler. At any rate, she had so many uses for it that its very insufficiency had caused her to play high in the hope of doubling it.
Hence, Lily is leading a life where she is literally swimming against the current; she has to "pacify" dressmakers and jewelers, which entails that she is in debt already. Someone who acts wisely and with common sense would not get themselves into situations of that sort. Moreover, she gambles at table as much as she gambles in life. This definitely would render her behavior as "impulsive"