Have you ever had a conversation at work that made you uncomfortable? Perhaps your boss told you about his marital problems or your co-worker started espousing her religious beliefs which are very different than your own. Why do these conversations make you squirm? It's because they could make your work relationships less professional and therefore awkward.
Not all topics are fair game when it comes to the workplace. There are some discussions you shouldn't have with co-workers, unless you also have a relationship with them outside of work. Subjects to avoid include politics, religion and anything else that could be controversial. Also avoid discussing personal problems. If you have relationships with co-workers that extend beyond "the office", not talking about personal things would be unnatural but exercise caution. Establish ground rules that set boundaries between your personal relationship and your workplace one.