4) so we can build a defence system against incoming asteroids 5) to provide a hedge against a planet-wide disaster (aforementioned incoming asteroids, global nuclear war, massive pandemic, runaway greenhouse, take your choice) 6) to get at the massive resources available in space - this one needs expanding. Yes, with current technology those resources simply aren't cost effective compared to what is available on Earth. But imagine a world where space-gun launches or a space elevator reduce launch costs down to tens of dollars per kilogram in orbit. That changes everything in the cost benefit analysis of space resources. 7) Science. We'll discover things that we never even imagined could exist. Who knows how those discoveries will aid humankind in the future?

Those are just some reasons that I could come up with quickly, but I'm sure there are others