For long, the art of the management of a software project is really an art, it's a trick that's difficult to master. The difficulty lies in the central problem the software project manager is faced with; appropriately named 'the software project managers' problem' [1]

Everyone effected by the project, direct or indirect, has something to say, again direct or indirect, and will do so. Everyone wants to get the best from this project for him personally, or for his (part of the) organization. It's the job of the software project manager to see that everyone gets what he wants, in one way or another. He has to "make everyone a winner" [1]

In this respect, the role of the project manager becomes that of a negotiator. The customer always wants to have it all for free, the user wants to have to greatest functionality, the programmer doesn't want to document, but wants to use the coolest compilers. The software project manager has to make them all happy.

[1] Boehm, Barry W. and Rony Ross. "Theory-W Software Project Management Principles and Examples." IEEE Transactions on Software Engineering. 15.7 (1989): 902-916.