I would also mention that there are many required properties that you want a authenticated key exchange (AKE) protocol to satisfy, e.g. authentication, key confirmation, forward secrecy, key freshness, secrecy on the session key.
What you want is allow Alice and Bob to stablish "session keys" for each session of communication. These session keys are established on some secret which has to be shared befor the protocol starts, i.e. long term symmetric keys or long term public keys.
Why do you want session keys to be established rather than using the long term keys for communication? One could say: 1. You want each session to be encrypted with a different key, 2. If you encrypt several sessions with the same key, it is subject to cryptoanalysis (for example, in cases of password based authentication where the entropy of the keys is relatively low), 3. You want to ensure that if some session key is revealed, it does not compromise previous communications. 4. Also, as here in the forum, AES requires fresh keys when establishment of communications.
Going back to your question, I would suggest that look at the properties I mentioned above and see if your solution satisfies them (Which I would say, your solution does not satisfy). This explain why there is a huge research going around Key exchange protocols and why the naive solution is not the best.
Hope this helps.
There is a very good book for key exchange protocols, called "Protocols for Authentication and Key Establishment", by Colin Boyd. Just in case you are interested on the topic :)