Hi - If your contract used the standard "Addendum of Clauses" form, then look on the first page. Most buyers/sellers agree to the first option. This allows the buyer to inspect the property and negotiate with the seller on repairs/issues that the seller is not required to address. This inspection option also allows the buyer to simply void the contract without giving a reason. There are two other primary inspection contingency that you'll see on that page -- a general inspection contingency and "as-is" clause. Take a look at your contract and speak to your listing agent.
If you agreed to a routine inspection contingency, should a buyer be required to provide a valid reason to void a contract? No - they should not. Here's why - the home must meet the buyer's own *subjective* criteria when making a purchase. The buyer may discover a feature of a home that's very important to him/her, but insignificant to the seller. Buyer and seller would rarely agree on what constitutes a "valid" reason to void a contract based on a property's condition, so ultimately it comes down to the buyer's opinion only.
Can the buyer use this language to simply void a contract because he changes his mind? Yes, he can. This is an unavoidable consequence of this language.
Again - take a look at *your* contract. If you don't have a listing agent, you should consult with your closing attorney about your specific status.
Good luck with your sale.