Butifsomehazardousmicroorganismsfromouterspacehitched a ridewiththemandmadeitintoourworld, wehumansmighthavenonaturalimmunitytothoseextraterrestrialmicrobes.
Andtheycouldslowlywipeourspeciesofftheplanet.
Ontheotherhand, ifaliensweren't comingheretomakefriends, we'd bedestroyedlongbeforewegot a chancetoofferpeacebetweenintelligentbeings.
Inanycase, wearen't readyfor a visitfrominterplanetarytravelers.