That’s Reassuring
We know that lunch is always a big deal around any office, and people are (rightfully or not) very possessive about their lunch and the ingredients they bring to the office. We also know that putting a sticky note with your name on the lunchbox isn’t necessarily helpful, and really, if somebody is after your lunch, there’s no way to stop them.
After someone from the office ate his lunch for a first time, a second time, and then a third time, he’s had enough. He realized there was nothing he could write on the food to prevent people from touching it, but there is something he could say that would make them regret it.