That is a tough question because it is so broad.
Possibilities:
1) The person cannot relate to kids, deal with kids, or just does not like kids. (Or cannot deal with noise or other child-created sensory input)
2) The person is too self-centered and cannot break out of this enough to do things a kid might like to do.
3) The person does not like a particular grandchild because...
a)The child is too different from the grandparent and cannot relate to him...
b)The grandchild is too alike to the grandparent in some way that reminds said grandparent.of his own failings.
c)The grandparent is just an ***.
d) The grandparent thinks the kid is bratty or undisciplined or has some other "moral" failing. (Whether true or not)
More contextual information would help narrow it down better. There are probably other reasons, but this is what came out of my brain, right now.