Answer
If the pericardial sac fits too tightly around the heart, then the heart would not be able to expand fully and the pumping action wouldn't be as effective.
If the pericardial sac fails to secrete pericardial fluid, then there will be an increase in friction between the sac and the heart, which would cause pain.
Work Step by Step
If the pericardial sac fits too tightly, then the heart doesn't have enough room to expand. Failure to fully expand will result in less blood filling and moving through the heart. The heart may pump faster in an attempt to move the same volume as normal.
Pericardial fluid is important to reduce friction. If there is no pericardial fluid, then the friction will be increased from normal and each heartbeat will be painful.
