Think of it like this: when two soldiers in boot camp are complaining to each other about their drill instructor, the difficulty of the situation, why the food

Think of it like this: when two soldiers in boot camp are complaining to each other about their drill instructor, the difficulty of the situation, why the food sucks, etc. - they are commiserating. It is a bonding over being in the same shared crappy situation, and having sympathy for that person because of it. The author is essentially saying this: a manager shouldn’t join in complaining about the job/company/personal problems etc. with his/her subordinates, because it sets up false emotional relationship expectations.