because we all know from experience that cats and dogs aren't just mindless creatures, most people don't have pet squirrels but if they did they would probably learn that they are similar to dogs and cats in that they have personalities too. the fact is that all (most?) animals do, which is why people get pissed off when other ignorant people just go and kill them because they don't think they have "feelings" or thoughts. i wouldn't be surprised if they felt fear the same way we do, because we are all animals and people forget that most of the time because our brains are so big and awesome
that's articulated badly, but you get the point