As can be heard in the movie "Contact...."
"If we are alone in the Universe, it sure seems like an awful waste of space."
Of course, given my tendency to play with people's mind, especially when we're venturing into conspiracy theory territory, I'd respond something like....
"If aliens invade? Did you know that there is some evidence to show that they are already here
? They use technology to look like us, and have infiltrated the top levels of all the major world governments, corporations and religions. I'd be able to give you sources, but most of these are classified, and they're monitoring all my e-mails in case I slip up."