What do you think about me using my coworkers microwave?
Did you ask to use your co-worker's microwave?
No, but I'm just using it to cook over the holidays. I need to go home to my family so I'll be back next week
Not asking others for permission to use their things, such as your co-worker's microwave, is inconsiderate. Talk to your co-worker first to make sure it's okay to use their microwave.
Fuck man, I don't understand why I can't use their microwave. What if they need it?
If you need to borrow someone else's belongings, you simply ask them if you can use it. Borrowing it without their permission would be disrespectful towards them.