The Energy Information Administration estimates that the U.S. energy consumption in 2007 was 101.600 Quadrillion BTU’s. The U.S. energy demand has been steadily increasing, and especially the fossil fuel based energy consumption grew to a level of more than 84% of total consumption.
Energy consumption by source in quadrillion BTU 
While energy demand increases, supply of cheap fossil fuel based energy comes to an end: We reached the era of “Peak Oil”, i.e. the point in time in which the maximum rate of petroleum extraction is reached and in which the rate of extraction will eventually diminish. As a result, energy prices will need to increase in the future.
World oil production (EIA Monthly) for crude oil + NGL. The median forecast is calculated from 14 models that are predicting a peak before 2020. 95% of the predictions see a production peak between 2008 adn 2010 at 77.5 - 85.0 mbpd. 
Even worse, our reliance on fossil fuels causes a significant increase in carbon emissions, which are a main driver of global warming. Cost pressures and global warming intensify the search for cheap, renewable energy sources.
Global carbon emissions by type 
1 U.S. Energy Consumption: EIA, preliminary information, http://www.eia.doe.gov/aer/pdf/pages/sec1_5.pdf
2 Original picture at http://www.eia.doe.gov/emeu/aer/eh/frame.html
3 Source http://www.theoildrum.com/node/3720
4 Source http://en.wikipedia.org/wiki/Image:Global_Carbon_Emission_by_Type_to_Y2004.png