I think that a challenge a project manager would face, especially with longer term projects, would be how to keep a professional atmosphere. I bring this up because if you are going to be working with someone on a project for an extended period of time, it is difficult to just keep conversation to just work related stuff. Forming friendships, I see it as a good thing. This is due to the fact that if you genuinely like what you do, and who you work for then you will want to do a good job. On the other hand, as a PM, it could be difficult to reprimand someone that you see as your friend. Therefore, PMs have to do a good job of building relationships with their employees while also being able to hold them accountable.