There have been many articles and studies and opinions written about this, claiming everything from genetics to politics to economics to obliviousness--most of which contains at least a little truth. I think that the real answer is more Darwinian. Like this:
Simplistically, evolution rewards organisms that are most well adapted to their particular environment. If that environment changes, then either the organisms adapt or they die out. One can apply evolutionary thinking to societies as well. Societies that are well suited for their environment (both physical and sociological) will fare better than those that are not. When they encounter an environment to which they are poorly adapted and they fail to adapt, then they will die out. Think Easter Island: their environment changed (probably self inflicted), they were not suited to the new environment and could not adapt, and their civilization crashed.
Coming from the other direction then, one can ask the question, "What can make a civilization die out?" Using evolutionary concepts, the answer is, "Whatever the civilization cannot adapt to." What this means is that once humans encounter a problem to which we cannot adapt, it will result in the demise of the species. Just like the dinosaurs. They were presented with a drastically changed environment to which they were not well suited and could not adapt. Thus they died off.
Eventually, however, some sort of problem or circumstance will arise to which humans are unable to adapt. If the problem is severe enough, it could extinguish the species. Or it could just disrupt societies and kill a significant portion of the population. Eventually this will happen. The difference between the eons dinosaurs thrived and the much shorter time frame humans may end up having is that the dinosaurs did not deliberately change their environment. They just went along with whatever nature handed them. Those changes were mostly slow, and different species of dinosaurs either declined or thrived as those changes occurred.
Humans, on the other hand, have been making enormous changes to the planet. As we make these changes, we expose ourselves to circumstances to which we may not be able to adapt. We keep changing the planet, more and faster, all the time: overfishing, desertification, genetically-modified organisms, nanoparticles, overuse of antibiotics, globalization, etc.. So far none of these has spelled our demise. But it's inevitable that we will make some change to which we cannot adapt.
An analogy can be made to antibiotic-resistant bacteria. Medicines and the body can fight off many bacterial infections, but the one that is resistant will survive, eventually winning the battle. Similarly, the problem that is most resistant to human intervention (or that we fail to fight) will win the battle.
Perhaps I am wrong about climate change. Maybe we will find solutions and save ourselves from the worst of it. There will undoubtedly be another circumstance that we will inadvertently create that will spell our doom. We are making so many changes so fast that something has to break that we cannot fix. I think that thing is global warming.