...who don't need even a single grape. They have the ability all on their own to make an overflowing supply of whine.
My point in criticizing the concept of "Life ain't fair" is that no one-liner bit of wisdom holds truth across the board. Yet our culture reveres the quote, and we pass these quotes back and forth, and often we do so INSTEAD OF specific analysis. In other words, we take the easy way out and repeat something we think is wise instead of acting wisely to formulate something more applicable.
"Living well is the best revenge"? Good quote. Maybe it is of some use here, too, but it doesn't suffice to answer Doc's issues. He's already been down the road of exacting other revenge, so offering him a wise quote about the benefits of not exacting revenge is... not exactly offering him anything of use here. Those enemies of his are safely tucked away in the past now, so there's no need to talk him out of further revenge. It's the revenge he's already exacted that needs care and attention.
Doc himself has too many quotes stuck in his head, which he has not looked past. In my view, some of those very bits of wisdom, which once served him well but no longer apply, are the very hurdles he is trying to cross now.
As for the grapes... finding out they taste like gunpowder may be a disappointment, but it is also a service. No destination is sweet enough to vindicate a bitter, struggle-filled journey. (Some of you may have a hard time with that if "heaven" doesn't turn out to be quite like you imagined it). The journey is the thing. How you get there matters more than where you are going.