Fossil fuels provided cheap energy for a couple of centuries. Because it was cheap, we built our lifestyles around wasteful energy use. But as time goes by we become more and more aware that we can't go on like this forever, or even for much longer. Fossil fuels are finite, so we can't use exponentially more each year as we have for centuries. They cause serious pollution, climate change, etc. Even the way we extract them, nowadays, causes enormous problems (fracking, tar-sands, offshore drilling, etc.)

We need to find a whole new way to make energy. And in the process, we need to learn to use less energy, because massive, unrestricted energy use has consequences we never realized before. If the human race is going to last another century or two, we have to learn ways to live without making the earth uninhabitable.

For the same reason non-humans need renewable energy. Renewable energy isn't dependent on a finite energy source.

An examble is biodiesel made from algae. As long as algae grows, there will be oil. But non-renewable fossil fuel sources are not increasing or growing.

Because our lifespans are short compared to geologic time. Fossil fuels are the chemically stored solar energy collected by photosynthesis over millions of years and the process is still happening. But it isn't acceptable for us to wait 60 million years for our next tank of gas.

Fossil fuels are NOT renewable and some other gases. We are rapidly using it up and if other sources are not found, our living will fall back how it was centuries ago.

Because one day fossil fuels will be gone and we won't be able to survive