1. They smell real bad (to us humans that is)
2. Their tentacles freak me out.
3. They're damn noisy, and their telepathic waves screw up my
4. They suck as girlfriends, even when they're sexy. My buddy has an alien girlfriend and if it weren't for the special sealed suit and helmet she wears, her skin would have poisoned him long ago on first touch. Not much contact there. They make poison dart frogs look tame.
5. Their food is positively disgusting, especially the stuff that's alive while they eat it.
... hey, what's that saucer hovering above my house? Holy cow, alien abduction! Now I've got the whole alien universe against me!