Survival of our species/Colonising Space
One reason is survival. At the moment we have all our eggs in one basket here on Earth. If nothing else, space colonization will be an insurance policy that will allow life to survive on other spaceships should something catastrophic happen to Spaceship Earth.
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. Freed from scarcity, Man tends to look for interesting challenges and ways to extend his horizons. What greater adventure could there be than extending life beyond our home planet and reaching out into the stars?
On a more pragmatic note, the resources in space of energy, material and living space are effectively unlimited. This will allow our civilisation to constantly expand while taking the burden off the Earth. Just as the New World held the promise of abundant gold and land to settlers of the Americas, space is a frontier filled with unfathomable wealth of all kinds. Simultaneously, it holds the best promise to help ensure our survival, and the most interesting way to extend our reach. It should be a top priority.