In Parshat Noach, we see an entire generation of people killed because Hashem saw fit that they be punished for their sins. The Torah says that the people were corrupt, degenerate, and violent, and Hashem thought it was necessary to start over. However, the question we must ask is: what were the specific sins of the Dor HaMabul?
The Midrash states that a major sin of the Dor HaMabul was that the men used to take two wives. They would have children with one and use the other for personal pleasure. Everyone will agree that this is a disgusting practice, but how was this seen as so corrupt that the people deserved to be wiped out by the flood? The answer is that taking two wives was the start of the decline of society. People saw personal pleasure as the number one priority. With this kind of mindset, the people became completely corrupt and degenerate, and the generation deserved to be destroyed.
When main “moral” of a society is self-indulgence, the society is naturally corrupt. Nowadays, all most people care about is pleasing themselves. Once our society opened itself up to accept some self-indulgence, morality levels dropped. Nowadays, the value people place on an item or activity is based on how much pleasure they can derive from it, not on whether it has intrinsic value or is the right thing to do.
The lesson to be learned is obvious. The important things in life should not be self-indulgences and what gives us pleasure. The things we should enjoy most are Davening and learning Torah. We should be working harder toward becoming closer to Hashem than we do toward having a good time. Once we permit ourselves to look exclusively for pleasure, even for just a short amount of time, we begin a downward spiral toward the corruption of society, as seen in the Dor HaMabul.