Tight coupling means difficult to refactor. Let us take a simple example of HAS-A relationship- A car HAS-A wheel. Now, suppose in future you want to refactor your code and want to move out the Wheel object out of the Car object, it will be bit difficult. That�s why HAS-A is tightly coupled.
Regards,
Anil Kumar Saha
SCJP 1.4
http://www.agilej.blogspot.com/