Maybe sympathy is an indication that humans recognize (or at least, believe they recognize) their power over other, weaker life forms? For example, if Al sees a cat being hurt in some way by Greg, then Al will feel bad for the cat because Al thinks:

-The cat is helpless (or at least, can't sufficiently prevent Greg from hurting it, and thus probably doesn't want to be hurt)
-The cat didn't knowingly do anything wrong (the cat might have eaten a plant that it shouldn't have, but only because the cat thought the plant was tasty, not because the cat knew it would anger Greg)
-So, the cat is not at fault and doesn't deserve to be hurt.
-If I (Al) were ever in a situation where I wasn't at fault and didn't deserve to be hurt, but was being hurt anyway, I would hate the injustice.
-Thus, I feel sympathy for the cat's pain, because I think there is an injustice being commited.

My opinion after a bit of thought. (Man, this forum is awesome!)