Why are we always in search of alternative sources of energy?

There are two reasons we must focus on alternate and renewable sources of energy. First, most of the energy sources we use now are harming the earth and all of its denizens, and second, most of the sources of energy we use now are finite.

Our primary sources of energy now are coal, gas, and oil, often referred to as fossil fuels, since they are created by the pressures of the earth upon fossilized remains....

There are two reasons we must focus on alternate and renewable sources of energy. First, most of the energy sources we use now are harming the earth and all of its denizens, and second, most of the sources of energy we use now are finite.

Our primary sources of energy now are coal, gas, and oil, often referred to as fossil fuels, since they are created by the pressures of the earth upon fossilized remains. All of these must be burned in some way to create energy, and it is in the burning that the earth is harmed and that the environment is polluted, which has health implications for people (and animals, although no one ever mentions that very much.) In China, for example, a country highly dependent on fossil fuels, people in some cities must stay indoors and cannot even see across the street because of the haze from the burning of these fuels. Carbon dioxide as well as other gases and dangerous particles are released into the atmosphere. These are referred to as greenhouse gases, and they disturb the earth's balance as more and more heat is held within earth's atmosphere. This is what creates global warming, or climate change. The implications of this are staggering. Warmer temperatures mean our icecaps are melting, our seas are rising, and agriculture becomes a very uncertain endeavor. Most of the animals we raise and the plants we grow to feed ourselves thrive in very narrow temperature ranges and as these shift, we cannot produce as we have before. The gases and particles also harms people's health, especially their respiratory systems. These are the fuels we need to replace in order to save the earth from their destructive forces.

At the same time, there is a powerful economic incentive to find other sources of energy because fossil fuels are not limitless. The earth holds only so much coal, gas, and oil, and when it is gone, it's gone. Thus, we need to have other means of generating energy, and it is renewable sources that we need to focus on. Renewable resources of the earth are sun, wind, and water. The sun comes up every day, the wind is always blowing somewhere, and the waves continue to break on the shore and tumble over falls. All of this is energy that can be captured, stored, and used to create electricity and heat. I believe that the cities of Niagara Falls and Buffalo are largely fueled by the force of the water that goes over Niagara Falls. We know how to do this, but we don't know how to do it as efficiently as we need to, and it requires significant investment to develop the technologies needed. Bear in mind that the technology and infrastructure for fossil fuel energies are already in place, so there is no large capital investment needed to continue with these, while there is a need for a great deal of capital investment to reap the benefit of the renewable resources. 
There is no question that fossil fuels are what have driven the development of the United States and many other countries, but when the Industrial Revolution began, no one had any idea how harmful these fuels were to the earth.  Now we know, and it is time to move on to a renewable resource revolution.