Leader member exchange theory

Leader-Member Exchange Theory (LMX) examines leadership dynamics in terms of personal relationships between an organizational leader and follower. However, this personal relationship exists in a professional setting. What is the difference between a Dyadic Relationship and a friendship? LMX Theory promotes the positive aspects of personal relationships between a leader and a follower. But, are there some leaders and/or followers that can't have very personal relationships with their co-workers, and is this good, bad, neither, and why?