Our eggs are currently all in one celestial basket. We need to try, desperately, to ensure that the human race does not go extinct (by fault of our own or otherwise). This means an alternative habitat.

We've gotten really good at terraforming the Earth (unintentionally) and killing each other (intentionally) in the past century. In particular, one man has the power to end the world, he has no political experience, and he is in the White House. If we have a nuclear holocaust, the only silver lining can be in human populations on other celestial bodies. Hence, it is in the best long-term interest of humanity to go to the Moon and Mars - to ensure our survival.