Photo Library "Repair" pop up keeps appearing after clicking. Why? Do I need to click repair for each of my 6,000 photos?

Photos will automatically repair the library, if it has become damaged.

Possible reasons for an automatic repair are:

• Repeated crashes - a crash of Photos can leave the library in an inconsistent state. Does Photos crash frequently or are you force quitting it?
• Accessing the library from a different user account. If you are sharing a library between users, the ownership of the internal files may change, unless the library is on a volume with "ignore ownership" enabled, and then Photos will always have to repair the library before you can use it again.
• Accessing the internal files in the library package from other application.
• After running a cleaning application to remove duplicates.
• If the library is stored on a volume with an incompatible file system format or on a failing drive or in a synced folder or on a network share, or on a volume used for Time Machine backups.

Where is your Photos Library stored?

You shouldn't need to repair a library very often. This is a useful article setting out when to repair a library:

https://discussions.apple.com/docs/DOC-250002264

You say that repair pops up after clicking...... what are you clicking? Normally the repair prompt only comes up when you hold CMD+ALT and then click the photos icon.