The answer is pretty obvious if we look at the prehistoric times. Like every other species, every other organism in existence, our primary goal is to ensure our own survival. We built fires to keep warm, so we didn't freeze to death in the wrath of winter. We hunted animals for food. We crafted tools and with them we built structures for shelter. Some even chose to live in caves but they weren't the first ones who had thought of this idea. Bears, hyenas and other animals that had been living there made life much more difficult for the troglodytes.
It was not long after that we started using the stars for our survival. We used the sky to get directions, to get hints about the weather depending upon which we took the necessary steps to prepare ourselves to combat the upcoming harshness of the weather. Soon, creation myths were created. The Sun, The Moon, and the stars were viewed as gods and goddesses. In order to ensure a good and bountiful harvest, tributes in the shape of sacrifices were paid. Religion is a major motivator in human history, the significance of which cannot be ignored, even in the slightest.
After all, what was there stopping someone from killing his fellow man aside from the idea of eternally damnation? The thought of the wrath of the Gods (or God, if you will) made one rethink his actions ten times. The concepts of hell and heaven made all the difference here. Why not do good when in the end you'll get to be in a place where everything you ever wish for, will come true? If people weren't doing good for the sake of doing good, they started doing good for the eternal rewards. Which, in my opinion, is as bad as it gets. A quote from my favorite TV show comes to mind,
If the only thing keeping a person decent is the expectation of divine reward, then brother that person is a piece of shit.
Of course, the obvious and consistent motivators through out human history are power and control. Numerous wars have been waged for the sake of power and control. There has never been a period in the human history where someone has not warred for the sake of power and control. Even the ruling Kings in history have been seduced by greed to squeeze out the last drop, which has often lead to their downfall and demise. Personally, I don't think that greed will ever be stop plaguing humanity. It has been programmed into our DNA, and we can't cut it out.
Today, as much as life has become easier, survival has become much harder. People who don't go to college are at a significant disadvantage to those who do. An increasing number of companies are hiring only college graduates and paying those who didn't go to college no mind. The minimum wage is decreasing, the number of jobs is decreasing. The average Joe faces a myriad of problems and hurdles in providing for himself and his family. So what is different today? The war for survival still goes on, even though life has become much easier.
Personally, I think that today, the scope has increased massively. In the ancient times, it was the question of survival of only an individual. Today, it is the question of survival of the entire human species. A number of massive problems are taking us right to the point of no return, beyond which our survival becomes doubtful.
For an organism, there is no motivator greater than the very prospect of death. You'd be surprised to know the lengths people can go to when they are involved in a life-threatening situation. Some times, they even exhibit almost superhuman strength. When it is certain to someone that the very idea of his death could become a reality, he becomes willing to go to extreme lengths in an effort to ensure that that idea remains only an idea, a thought. As long as he has strength, he will fight it. To ensure his own survival, one is willing to kill others which is why we have built and used weapons of mass destruction. Inevitably, these very weapons of mass destruction will be our undoing.