Why does Outlook ask me "Do you want to save changes?"

While it can be annoying, I know of no way to turn that off globally. But you can do so per-sender.

The real question is why is it asking in the first place? There are actually valid reasons. And yes, there are times when you might want to answer "yes", and times when you might want to answer "no".

We've all seen this message from Outlook:

Outlook displays this when it prevents pictures from being displayed in an email message, which is the default behavior. The privacy issue that this behavior addresses is that HTML formatted email can reference pictures that are stored out on the internet. Because of that, merely fetching such a picture to display it could cause the sender to know that you had viewed your mail.

Outlook lets you choose to display pictures on a per-message basis, or you can add the sender's email address to a "safe senders list". Right click on the message described above in the email message, and you'll see this pop-up menu:

• Download Pictures causes the images referenced in the email to be retrieved from the internet, and displayed.
• Change Automatic Download Settings... brings up thisdialog:

  

  which allows you to control when, and how, Outlook automatically downloads pictures.
• Add Sender to Safe Senders List adds the email address of the sender of the email to a list of known "safe senders", and the images for this message are downloaded. Images will be automatically downloaded for future email from senders on this list. This same list is used by the junk mail filter to identify senders which are known not to send spam.
• Add the Domain @example.com to Safe Senders List adds the entire domain of the sender's email address to the list of known "safe senders", and downloads the pictures for this email. Future email from any email address on this domain will be considered safe, and pictures will be downloaded automatically.

Now, what about this message:

When Outlook downloads the images for an email message, it stores them with the message. Now, if the images were downloaded automatically, then they're just part of the message to begin with. On the other hand, if you open a message where pictures have not been downloaded, and then explicitly say "Download Pictures", Outlook fetches those pictures, and adds them to the email message. Technically, that changes the message. Whenever you make any change to a message you've received, Outlook needs to confirm that you want to keep those changes when you close the message.

You most often want to answer "yes". That way, the images are kept with your message, and the next time you view the message they don't need to be downloaded again.

You might want to answer "no" to save space, for example. As I said, the images are kept with the email message, and if you save the updated message with pictures, it's naturally much bigger than without the pictures. If you don't plan to view it again, or you don't mind downloading the pictures each time you view the message, this might be a viable alternative.

The best, and really the only way, to avoid being asked to save changes in this case, is to add senders you trust to your safe senders list. That way their pictures are downloaded immediately and become part of your original view of the message. You don't have to explicitly download the pictures causing a "change".