There are never enough resources available to efficiently execute all of the projects that have been initiated! The resource pool owners (line/functional managers) have extremely difficult decisions to make in this multi-project, shared resource environment.  They are constantly bombarded with request from Project Mangers for the perfect skill match for their projects.  There are limited numbers of resources available and and even shorter supply of resources with the desired skill levels.  When a Project Manager comes to request resources he/she better be prepared to communicate the specific project's organizational priority.  Justification of why your project warrants the assignment of needed resources must be confirmed.