Understand That People Are "Where They Are"
This has become a pet statement of mine. Having run a service-based consulting company, I sometimes had to deal with difficult people. Occasionally, my consultants would get upset by the statements and attitudes of these clients.
I would remind them, "People are where they are."
What I meant by this was simple. Often, we don't know the situations surrounding that person's workday or life. I'm not making excuses. If you have a rotten day, I still believe you should treat people with respect and kindness. However, my admonition to act professionally in all circumstances does not mean that all people will. In the overall scheme of building your career, curbing negative emotions just makes good sense—even when the other person is out of line.
When dealing with people, you need to understand that there is a difference between what is the case and what you want the case to be. You need to deal with people based on what is actually occurring.
When I say that people are "where they are," I am advocating a view that puts responsibility entirely on you to act appropriately in all situations, not the other guy. If I were speaking to "the other guy," I would be telling him the same thing. However, you are the one reading this, so you are receiving the ten-minute lecture.
When it comes to furthering your career, this is vital. When (not if) you find yourself working for or with difficult people, your ability to advance your career is dictated by how effectively you deal with them. Your desire that difficult people should act differently has little bearing.
A counselor told me once that we typically "die in our affliction." What he meant is that people seldom change. That includes you. The only thing you can control is what you do and how you react. Understanding that some people are inherently difficult can help you maintain more objectivity during your day-to-day dealings with them.