Disclaimer: Not mine.
It all builds up after years on the Job. Builds up to the point where faces that use to have names become just faces, eyes staring at you through the mists of time. Builds up to the point where the names of murderers and rapists fade to ink on a report that's in a file room somewhere. Builds up to the point where nightmares are every night, but you can generally go back to sleep.
Where they all blend together so they become dead bodies looming out of the darkness, greeting you with dead eyes and whispering things about broken promises.
After years, it all just builds up.