Energy Security for a country means being certain of two things: being sure that you have access to energy; and that its supply can't be used as a political threat against you.

Given the risks of climate change, habitat destruction and pollution, why are countries even trying to develop new sources of fossil fuels such as shale oil/gas and tar sands and using methods like Fracking?

One big reason is "energy security". The modern world relies totally on cheap energy to function. So as the supply of viable sources dwindle, those countries who can still supply it gain great political power. Countries without their own supplies become dependent on those that do. And the biggest source of profit in business is to have control over something that everybody wants, but only you can supply.

As conventional oil and gas wells have been drying up, the most of the known remaining supplies have been in middle eastern countries (oil) and Russia (gas). These have been politically unstable for a long time, meaning that the countries who buy fuels from there are concerned that they may not be able to rely on access to this cheap fuel for much longer. So developing alternative sources, either in your own country or in one that you can rely on to be friendly, becomes politically desirable.