Why are some photos harder to delete?

Was going through our old photos and deleting and like a lot weren’t to difficult just the standard level, but then I found ones that were like accidentally screenshots I took where she was smiling at me over FaceTime, or this photo we took in an old Polaroid, or this selfie I took where she wouldn’t let me leave for work that morning. It would be easier I feel like if we hadn’t been so compatible, but we broke up due to circumstances and those photos where she’s smiling at me are so much harder to delete