Understand rights versus license. Rights are privileges you enjoy simply because you are a member of a class. Copyright is a right the attaches because you have a completed creative work. Some rights are transferable, but other than that, you retain them indefinitely. License is a limited privilege that is granted as a result of a contract, and remains in force for the duration of the contract, provided all the terms of the contract are upheld by both parties. Publishing is a use license between the copyright owner and a party in a position to increase the value of the copyrighted work, through a mutually beneficial contract. Exclusivity is a common contract term, to ensure that the publisher has a chance at a fair return on that contract.
Please don't lose heart. There are so many reasons for a rejection letter and more than half have nothing to do with you. The agent might be too busy at this time, or they might not want to take on new clients, no matter how wonderful their work.
