If a quantum system interacts with a "big" quantum system, you have dephasing.
The models of decoherence all have this atog aproach to them, about what is to understood of the interaction of the quantum state with a bath.
At which point does the bath character of the "bigger" system strike? In which way are the systems involved in decoherence fundamentally different? Why is it clear that a bath will behave in such and such way, that the phases vanish?