Why is this happening when I import photos?

"What can I do to my photoshop so that it doesn't lower the image quality? "

What you mean by lowering quality? Are you losing contrast sharpnes or something else when photos are imported or converted to another profile? You can view your photos at 100% magnification level always when judging about quality. Keyboard shortcut is Ctrl/Cmd + 1. 

Daniel AP wrote:

“I just tried that and that pop-up box didn't appear! How come it's different when importing?” 

When you open, there’s only one color space to be concerned with: The color space of the file you opened.

When you import a document into an already open document, if the two documents have different color spaces, you have to tell Photoshop which one should win.

There is a way to avoid those alerts, because they come up only if your Color Settings are set to warn you about color profile mismatches. If you have not seen those alerts previously and they started appearing all the time, the Color Settings might have gotten changed somehow. Choose Edit > Color Settings and make sure the current Color Settings are appropriate for your workflow, especially the settings for Profile Mismatches and Missing Profiles.

If you are not sure which Color Settings preset is the right one for your work, from the Settings menu choose the General Purpose preset for your region. The General Purpose presets have the profiles mismatch warnings turned off and simply convert between any mismatched profiles without asking.

Importing is very different (from Opening). You already have a document, an art board, a colour space, a resolution and a size in pixels. When importing you have to make sure things match or coordinate in the way you want, so you need a lot more planning. 

Do you have to import? Open is simpler to get right.