For Jody's grandfather, the "crossing" represents his youth and opportunities to be both protector and conqueror.  In Grandfather's day, the frontier was still open.   There were wars to be fought, land to be acquired, adventures to be had.  He remembers when he was the leader of the party heading West, into the danger of the great unknown.  It was he who had to protect his group.  He tells Jody,

"I was the leader, and I kept my eyes open.  Know why?  Well, just the minute the people began to get hungry they'd start slaughtering the team of oxen....The leader of a party had to keep them from doing that."

And,

"Did I ever tell you how I wanted each wagon to carry a long iron plate?....  I thought that if every wagon carried a long iron plate with rifle holes, the men could stand the plates on the outside of the wheels when the wagons were in the circle and they would be protected."

Jody's father, Carl becomes impatient with Grandfather and says, (thinking the old man is out of earshot):  "All right!  Now it's finished.  Nobody wants to hear about it over and over."

Carl does feel badly that Grandfather hears his angry words.  "I'm sorry I said it." 

Grandfather looked sidewise.  "I'm not being mad.  I don't mind what you said, but it might be true, and I would mind that."

It is hard to realize, for anyone, that their "glory days" have passed.