Let's say you run across an alien during a walk in the woods. How could you ever tell whether the alien was friendly or not? Aliens might act friendly, right up until the time they put you in the big alien stew pot. Likewise, friendly aliens might use deadly force and extreme caution to make sure you pose no threat. Would you want to be sending "Howdy There! We're new in the block! Drop by for some free pizza!" messages to a civilization 20 light years away?

I must have stopped reading it fairly early, because looking at the cover I don't remember any of it. I remember the first book, however -- that super guy they brought back for Ender's training and the planet and all. It's been a few years, though. Good thing I kept all the books.

Apologies for the poor memory.

The problem is that cats and dogs have opposing body language; when a dog acts excited and happy, the cat interprets that language as aggression. For example, a cat with its ears down and tail wagging is a cat that's about to fight. A dog doing the same thing is being submissive.

Of course, this all assumes that any intelligent extra-terrestrial life even has any sort of body language or capability to communicate that we can even recognize.