Well, it's not genetic or anything. It's just that the Deegans are very family oriented, so Dominic probably knew his grandpa fairly well, and, heck, probably looked up to him. Here was a man who shared interests with Dominic, rather unlike his father who was all about the sword and looking hot. Not only that, but Grandaddy Deegan was a man of power, someone who could and did make other, lesser men his playthings, a position no doubt appealing to Dominic, who was constantly bullied in school, unable to defend himself. Though Dominic loves, and has a good relationship with, his father, here is his true male role model.
It's all coming together.