Throughout time, humanity has been obsessed with the apocalypse. Many generations have thought it would happen in their time. Now, most people would agree that different generations of humanity, have thought that the apocalypse would be coming relatively soon. I believe there are two main reasons for this for which this may have happened for which many other sub-reasons may develop. The first reason is because the mere act of being a mortal being, having self-awareness, and realizing that you have a finite lifespan has a devastating effect on thinking about not existing. One reason people may believe that some part of them exists after death would be to help alleviate the self-aware, mortal dilemma. Another reason would be if people were so attached to their self-image, they could not conceive of what their life would be like without a higher expectation in the work force.
How can someone find meaning in activities they engage in, jobs they perform, or friendships they make when they will cease to exist within 100 years? Well, religion and spirituality attempt to address these subjects, but without them, it is a very daunting task. In fact, even after religion and spirituality have addressed these subjects, many people still feel uneasy about them. Religion and spirituality require a certain level of faith, and the academic section of our society promotes skepticism without evidence, something that cannot be provided by esoteric beliefs.
Another way a person could deal with the self-aware, mortal dilemma would be to assume that the entire world will end in their lifetime, thus taking away the challenging task of imagining a world without themselves placed within it.
Which brings me to the other main reason why people have thought and still think to this day that the apocalypse will be coming in their time. A little thing called ego. I'd be willing to bet that you could make a fairly accurate graph with increasing egos correlated with higher proportions of people believing that the apocalypse will occur in their time. If one has no ego and doesn't think they are important at all, it is not hard for them to imagine a world without themselves in it. But, for the person who has a very large ego, it is hard for them to even imagine people enjoying themselves without their company. And wouldn't it be a more self-serving belief to think that one will see the end of all existence in their lifetime than to think one will die without the world even noticing?