for the same reason solitary confinement is scary. or...being the most important being in existence is a lot of pressure scary. It depends on how you look at Fermi's Paradox, but the idea that we are "alone" left to our own devices, in this thought exercise, is scary. Scarier than an impending war between humans and aliens...because Fermi's Paradox already answers that question...they are either too far away for contact to ever happen, we'll be the first - so they don't exist yet, aliens are simply observing us - zoo theory, or they are hiding for some reason (one that wouldn't lead to them showing up like its clobbering time either)...or...well...we're alone.