You’re not entitled to other people’s belongings. 

This is a pretty simple concept, especially at work. If you noticed that your coworker was drinking a coffee, and you thought to yourself, «I could really go for a coffee right now,» you wouldn’t just take their coffee. You would get up and make your own. That is, unless you’re the coworker in this story, and you have a different relationship with other people’s stuff. Nobody likes to feel like they’re being taken advantage of, no matter who they are. Everyone wants to feel valued and considered, and like they’re being respected by the people around them. When you’re at work, you want to trust the people you work with. You want to know that they’ve got your back, and that they’re in it with you. 

Your relationship with your coworkers is an important one, and not to be overlooked. You spend so much time with these folks that you ought to think about your relationship with them. You need to get on with them somewhat well, or else you’ll spend so much time regretting it. It’s much easier to coexist with your coworkers than it is to have beef with them. But sometimes someone’s behavior is so upsetting that you have no choice but to have beef. People sometimes have unruly and unrelenting behavior, and you can’t help that; you can only respond to it.