My grandfather would come over and the cocker spaniel mutt that we had when I grew up at home couldn’t get enough of him. Jumping at the door, crying and whining to say hello, couldn’t wait for him to sit down so she could climb up into his lap for attention. Oh man, it was like the rest of the world ignored her and he was his only attention when he visited. And she was a pleasant dog for everyone else, but oh man, when he came over, it was heaven on earth for her.
Then after she died, my parents got another cocker mutt, and she didn’t like Grandpa at all. She would cower around the house, keep her distance, never let herself within ten feet of him. But everybody else was fine, she didn’t mind anyone else at all.
Dogs can be silly like that.
Sorry about your loss. Diego sounds like a wonderful companion.