This is an important question because the statement is quite true, and there are certainly pros and cons. Managers and employees should have an awareness of both, in order to make good decisions about relationships of any kind in the workplace.
Outside of family, most of our relationships are based upon proximity and/or commonality. Once we leave school, the people whom we are most proximate to and the people we have a great deal in common with are often the people where we work. Most people spend forty hours a week at work, so having friendships at work makes a great deal of sense from even a practical point of view. People are so busy today, and if we can make friends at work and it spills over into our personal life, it makes life easy and convenient. There is a great deal in common, and strong friendships can be forged this way. People fall in love, too, with people they meet at work, and on the face of it, there is nothing wrong with this. But looking beyond the obvious advantages of proximity, commonality, and convenience, problems can arise.
First, a friendship that is work-based and personal can make people unproductive. They can tend to lose sight of their job focus and spend too much time on the friendship. Second, friendships that are like this, if they are obvious, can have a tendency to make others feel left out, which is very bad for workplace morale. It's much like the third grader who invites only half of her classmates to her birthday party. Third, a romance might actually be forbidden as a matter of company policy. There are companies that prohibit this, between co-workers, or more commonly, between a supervisor and an employee. And that brings me to the fourth reason there can be problems, which is that a relationship between a supervisor and an employee is ripe for charges of sexual harassment. This is a relationship with an implicit imbalance of power, and it is human nature to take advantage of one's power in a friendship or any other kind of relationship. Fifth, when a friendship or romance goes bad or even concludes amicably, there is often a great deal of awkwardness in the aftermath. Some people feel a need to actually change jobs or transfer to other departments, which is a disruptive force in the workplace. Finally, even when people leave jobs for other reasons, the friendship in one's personal life can become awkward, particularly when it is apparent that two people had only the job in common and don't really like one another all that much otherwise.
All things considered, it can be wonderful to make friends at work and incorporate them into our personal lives. But we must have some awareness of any work policies that might prohibit this, and we should be fully aware that problems can arise. That awareness makes us a bit more careful about our workplace friendships.