One very reasonable expectation that we all have of others is that they respect our physical and emotional boundaries. Even people who are otherwise easy to get along with can become irritated and impatient when someone oversteps their boundaries. Understanding the concept of boundaries can be particularly important in the workplace.
Boundaries can be physical or emotional - that is, what you will and will not tolerate in the way others treat you or talk to you. People who across these boundaries without permission can seem difficult. For example, a subordinate crosses your boundaries when he walks into your office uninvited, cuts you off in meetings, sends out department-wide directives without consulting you, or makes unauthorized purchases. He becomes a classic difficult subordinate. Or a competitive co-worker at or near your same level on the organization charts ventures into your territory. He becomes a classic difficult co-worker.
Some people will test you at every turn, even creating opportunities to see how far they can go. Enforcing your boundaries, without going to the extreme of shutting people out entirely, is important if you intend to earn popular authority with peers, subordinates, and even superiors in your company. A reputation for getting walked all over won't earn anyone's respect. The alternative - keeping to yourself - won't ingratiate you with your group or organization either. The middle ground is to be politely assertive.
It can be even more challenging to deal with a boss who seems oblivious to the boundaries to which you are entitled - a reasonable amount of privacy, autonomy, and self-direction in doing your job. Although he may feel that his authority gives him carte blanche, a trusting relationship can't develop if you are constantly worried about him popping up in your physical or emotional space without warning. Moreover, your problems with your boss can spill over into your relationship with your subordinates.
You have the authority to enforce boundaries as the head of your group or department. Doing so can help you avoid difficult behavioral problems. Enforcing boundaries that make sense to people and help build and maintain a comfortable working environment will eliminate confusion and embarrassment.