Because of statistics. If we can find life, potentially started independently or pollinated in a couple of planets close to each-other, then life is everywhere and we may be in a dark forest of intelligent or hidden native predatory beings waiting to wipe us out if we squeak or show up elsewhere. If it's not as frequent, then we can keep exploring to look for it and more human-habitable spots.

I have a premonition that even if extraterrestrial life is discovered, and even if intelligent, hostile extraterrestrial life is somehow discovered, humans are not going to leave it alone.

After all, everyone involved can just say it's inevitable! If we don't do it, someone else will.