Why an onMessage call in Message-driven bean is always a seperate transaction

The MDB is stateless and inherently each message is unique with respect to the MDB. 
Each message needs to be processed independently. Hence the need for separate transactions

Message driven beans cannot be instantiated by a client application. The container creates the mdb instantiates an mdb each time a message is available on a jms queue. Hence by default each message is processed in a seperate transaction, whose life is as long as the message processing isnt complete. In case of failure of processing this message, the transaction is rolledback and the message put back onto the queue.