Man is a curious animal. There is so much out there to learn and we have the capability to do so, not to try would be wrong.
1) we probably ARE alone, realistically, as we are unlikely to ever come in contact with another intelligent civilization, but SO WHAT? There is still a lot to do.
2) don't even CONSIDER the possibility of us "using up the Earth". We need to take care of Mom Earth, as we don't have a spare.