Just a little wondering..

My old boy (14 this boxing day) is a very sweet and gentle soul, he loves everyone. Now, I have 3 granddaughters and he's very invested in them and when they're here with us he needs to be involved in whatever they're doing BUT he is absolutely obsessed with the middle one, and we can't figure out why? I mean our son came home earlier with a bag of her clothes that he was gifting to a friend, which still smelt like her I guess, and he was running all over, crying, pawing the bag and just generally being extra. So I guess.. why? Why has she been singled out for such devotion? Anyone have any similar stories? They've all known him their whole lives but for whatever reason she's just.. special to him? Edited pic to show this is about as far from her as he ever is.