Why does my cat sleep outside the puppy's room?

My puppy is four months old now and Binky (our 15 pound cat) isn't really too fond of her, but when we put Pepperoni (the puppy) in her room he likes to sleep right in front of the door. He does not lay there otherwise. He's only slapped her twice and has hissed and growled at her a couple of times, but other than that there have been no problems. Is it possible that he actually does love her but is simply annoyed by her energy? I'm ready to have both of them together in the same room with no fears of him attacking her, but he's a grump! I would also like to add that the puppy, although interested, does not bother or chase him but if she ever did...well, like I said...he's a grump and I'm scared he'll seriously hurt her. What do you think?