Odd thing between shanks and monkey D. Garp

It's no secret that garp, as Luffy's grandfather blames shanks for all of Luffy's troublemaking, even though it's obvious that it comes from his blood line.
However, garp fails to notice that shanks saved luffy from drowning, and that he also saved Coby from akainu at marineford. It's funny how he keeps hating him, but shanks protected to precious people to garp.
I always saw Coby as the grandson that garp wishes he had.
Any thoughts related to this?