Why are they called pet peeves, even if they have nothing to do with animals?!

So I have been at the new job for officially one week now, and I already have beef with somebody here. I typically bring my lunch in everyday, and will often bring in a beverage (Red Bull) and a breakfast item. Last week, on only my second day on the job, I went to get my afternoon pick me up in the kitchen, and somebody stole my Red Bull. Now, I want to preface all this by saying that I am far and away the youngest person here. One would think adults would know enough to not take something from the fridge that isn't theirs. However, when I went to warm up my frozen breakfast sandwich mid-morning yesterday, I noticed that one of the two was missing. Really?! Somebody opened up the box, helped them self to one of the sandwiches, and put the half filled box back into the freezer. People, if you didn't bring it in, it isn't yours, keep your damn paws off it. This sort of thing infuriates me, especially when you are working with adults. I just don't understand why people do this. I guess it is the cheapest form of grocery shopping, but have some common sense. Now that this has happened to me twice in one week, looks like I might have to put a note on the fridge, or rig up a camera in the kitchen to try to catch the dirty pirate hooker in the act. Hopefully this ends now, so I don't have to bring in my own personal mini fridge, I'm just saying...