I want the client to respect and not resent these types of contract clauses.
That's just not going to happen.
The client bought something from you that they thought was good value for money, only to later find out that it lacks an important feature (the practical ability to make modifications) which they assumed would be included as a matter of course. Even if your contract is completely fair and reasonable, and even if it's totally the client's fault for not reading it carefully, of course they're going to be pissed off.
Looking at it from the client's viewpoint, you've basically managed to vendor-lock them into paying you for any changes they need made to the piece. This is generally neither a pleasant nor a financially sound position for the client to be in, especially if your contract does not specify how much you'll charge for such services in the future.
If I were your client, I'd be weighing the expected total cost of several possible choices at this point:
Letting you keep the files and just paying you every time something needs to be modified (and hoping that you won't lose the files, go bankrupt, get run over by a bus or just get greedy and start jacking up the price).
Paying whatever you ask for the native files, and just writing it off as the cost of a lesson learned.
Getting another designer to take whatever files the client does have and reconstruct something editable from them.
Just getting the whole thing redone by someone else from scratch.
In any case, personally, in any future design contracts I made, I'd be sure to explicitly request native files ("the preferred form of the work for making modifications to it", to borrow a phrase from the GPL) as part of the initial quote to prevent this from happening again, unless I was sure that the work was a one-off piece that I never intended to modify or partially reuse.
Anyway, to answer you direct question, you can't and shouldn't explain the value of native files to the client — the only real value they have is what you can squeeze out of the client because they don't have the files, and the client will not be particularly interested in hearing about that.
What you can do, instead, is try to find a compromise that both you and the client might be satisfied with. For example, if your original files really contain some "trade secrets" that you don't want anyone else to see, consider whether you might be able to provide the client with a simplified version that doesn't include the secret parts, but is still sufficiently editable to meet the client's needs (which might include, say, replacing an old embedded logo with a new one, or editing the text to fix outdated contact information).
(Just to be clear, I'm not suggesting that you should provide even such a simplified version of your native files for free — after all, they provide added value to the client, and even just preparing them costs you time and effort. Still, you might be willing to part with such simplified files for a price that the client might consider more reasonable than, say, three times the original project cost.)
If your design uses third-party assets such as commercial fonts, it's also generally reasonable to leave them out of the editable version and inform the client that they'll need to license them separately from their respective vendors. After all, that's something the client can do, and even if it's expensive, the money's not going to you, so you have no perceived incentive to artificially inflate the cost.
You might also consider offering to do one set of edits for free, assuming it's a reasonably small job, as a sign of goodwill and for the sake of good customer relations. Not only will it make the client feel like you're "meeting them halfway", but it lets you keep your "lock" on the client, while at the same time giving them extra time to consider the matter, hopefully with a better impression of you than they currently have. (Of course, you should only make such an offer if you're reasonably confident that it'll pay itself back later. I'm not saying you should work for nothing, just that sometimes it may be worth taking an initial loss to retain a client.)
In any case, if your contract with the client does not already include a "support plan", now would be a good time to suggest one. The client will be a lot more comfortable staying with you if they know in advance how much you're going to be charging for changes, and also how much they'll need to pay for the native files if they decide they want them later.