Spring AOP creates dynamic proxies based on the interfaces that the class implements. In your Hello class, there is no interface that is being implemented, hence Spring AOP cannot create a Proxy for you.
Also, just another reason why you should be coding to interfaces.
However, you can have Spring AOP use CGLib to generate proxy subclasses of Hello. I just would recommend coding to interfaces, and getting used to doing that while learning Spring. You will only be helping yourself here following that best practice.