We've had our (rescue) dog 4 years now. He has suddenly become obsessed with one of our cats. He licks her head and back, then whines quietly, watching her sleep. When she gets up, he sometimes follows her to give her a lick. When he's whining, he's looking very alert, like he's watching over her.
She's quite tolerant of it (swipes him gently when she's had enough), but is there a reason? She's spayed. He's entire.