Dinosaurs are those fascinating creatures that every small child loves, and which stirs the imagination of old and young alike. The last large mass of dinosaurs became extinct around 65 million years ago, give or take a few eons. How and why did they "suddenly die off"? The theories on how the dinosaurs became extinct have varied over the past two centuries, ever since the remains of these creatures were first identified and classified. The theories are constantly changing with new discoveries and new technology. Let's look at a few theories and may develop some of our own.
One of the first theories was that the dinosaurs became too large and literally ate themselves to extinction, there just wasn't enough food to support animals weighing hundreds of tons on the land. Also considered was that as the herbivorous dinosaurs became bigger, so did the carnivorous ones and they ate more and more of the others as they were easy to find, so both went extinct. This idea was first proposed in the 1800s and sounds reasonable but ignores basic biology. The problem with this theory is that animals only eat up to their food supply, then stop breeding. It is a control found in all ecosystems around the world, it is why elephants are disappearing in Africa as man removes their forests. The same should have applied to the dinosaurs, they only eat up to their food supply.
Disease was also considered as a reason for extinction. The theory is that a bacterium or virus evolved for which the dinosaurs had no resistance. Diseases wipe out small inbred population of animals all the time and even bigger populations can be devastated by disease. According to some historians 70% of the North American Indian population was killed by European diseases, not bullets. New diseases do evolve. The problem is the disease should not have killed everything, there is variety in nature and some should have developed immunity. It also should not have have spread globally without a rapid transport system.
Shifting climate is another theory for the extinction of the dinosaurs. It is known that the axis of the world shifted for some reason and weather started to change, everything becoming colder. The shift could have been caused by a large passing astronomical body or even shifts in the earth's core such as happened recently in the Pacific Plates which resulted in tsunamis and earthquakes. It is possible that this could have killed many of the dinosaur species in more northern (or extreme southern) climates near the poles as food disappeared while the dinosaurs needed more to keep warm. But we now know that some dinosaurs were actually tolerant of cold weather and then there is the problem of the dinosaurs in the tropics, why did they disappear? Evolution usually compensates for environmental changes unless they occur rapidly.
Then came the Meteorite/Comet Theory of extinction. It is now believed, and there is geological support, that a giant astronomical body impacted the Earth about 65 million years ago, causing sudden cataclysmic destruction around the world. It shifted continental plates, triggered massive eruptions and changed everything at once. Large animals lost their food supply and in some cases poisonous air, comprised of dust and gases were released. The giant herbivorous dinosaurs died and the carnivorous ones soon followed. Smaller creatures with slower metabolisms could survive on less, and so they took over while the world recovered.
A comet would have, combined with the numerous volcanic eruptions, caused gases to be introduced that would have poisoned the dinosaurs, looking like disease. The food would have also disappeared suddenly, wiping out normal biological controls. The climate would have definitely changed suddenly, not allowing time for evolution to compensate. And thus all the theories could be correct. The truth is that, unless a time machine is developed, no one will ever know truly why the dinosaurs went extinct.
Of course maybe it was it aliens from other space clearing the planet for human species to evolve!