Colonising Space/Intro
One reason is survival. At the moment we have all our eggs in one basket here on Earth and if nothing else it will be an insurance policy for our species, and other life. It is also the next great step for life inhabiting a new environment and spreading itself ever wider - something it has been doing for the past 3.7 billion years.
The resources in space in terms of energy, material and places to live are effectively unlimited which will allow our civilisation to constantly expand while taking the burden off the Earth. Apart from anything colonising space will be a massive adventure. Freed from scarcity, Man tends to look for interesting challenges and ways to extend his horizons. Seeing as extending our civilization beyond our home planet and reaching into the stars will be one of the greatest adventures humanity could ever undertake, while simultaneously doing the best thing to help insure our survival, it should be a top priority...