IMO the King loves his grandchildren, but I think he feels more at ease with Elena´s and ´Cristina´s children.
He defo does, the question is why. The other grandkids are older and can use their brain, not everybody is into toddlers. JC looked uncomfortable on almost any occasion involving the Asturias' children and there are very, very rare occasions when he gets approachable towards Letizia.
The worst example is when Leonor was born, on all pics JC looks like a copy/paste person and even on the all important pic with the three generations, himself, Felipe and baby Leonor, there is this urrggghhhh look on his face. I wonder if Leonor was a boy, would JC, clearly a man's man, be all over that child in public.
The Easter mass has always been a public display of non-affection towards the Asturias, this year is not an exception, JC clearly decides against acting his part as doted granddad, what he could easily do.
We've seen Prince Claus backing Maxima on her engagement day or Henrik hugging Mary - genuine or not, the gestures have been there, in front of the cameras. It's only JC who is refusing to do something similar, showing people that he is in full agreement with the CP couple or the daughter in law.
I agree with dKaren that if the Asturias had the choice, they wouldn't be there. But at the age of 42, Felipe and his family still only answer to one person, the King, what will certainly be frustrating at times. So if JC wants them to be there, they will come and have to endure his quite powerful gestures, his ignorance towards his successor and his family, and a facial expression between desinterest, annoyance and disappointment on the situation.
As I said before, not everything is as well as Sofia sr's smile wants to make people believe, and it always shows at Easter (and of course on National Day, when the Infantas join their father in his attitude).