I've come up with one of these a while back. The only way to tell what makes something happy is what makes them do that more. Thus, anything that can't learn either isn't sentient, or, if it is, it's equally likely to like or dislike anything you do.

Also, anything that would be less sentient than a tiny piece of your brain. It might be sentient, but it's less sentient than you. If there's enough of them that can be a problem, but just make sure there aren't that many.