synchronization means "the hack of least resistance". It just works and most everyone understands how it works, but it has some weaknesses that could affect your design under heavy concurrency. Not least of which is any client effectively has direct access to your object's synchronization lock meaning if they grab it and hold it other clients can't. In other words, the locking implemented by default synchronization effectively "publishes" you object's internal locking mechanism. Yuk. Its like setting yourself up for self-inflicted denial-of-service.
If you make the reentrant lock internal to your class (or just don't use the synchronized but do something like synchronize on some internal object that you new up in your constructor everywhere that you want synchronization), you remove this side-effect of publishing your internal locking mechanism, with the added complexity of you having to remember where to apply this internal synchronization as your class evolves.
