Equation
Lieutenant General
People are less psychologically affected on a personal level with increase in distance to enemy and abstraction, and directness of combat method employed. The grunts would feel much stronger personal emotions against opponents who were trying to shoot at him personally or in close contact combat than a personnel on a ship, who would be inclined to think of the action as more ship against ship rather than men on ship against men on ship.
On Killing by Dave Grossman was an interesting read.
Lt. Col. Dave Grossman was a speaker at Ft. Lewis at the time when I was training there and he was an amazing person and officer. One of the best motivator speaker I've ever met, and he knew how to rile the cadets at the time for the hardest part of the camp (platoon ops). I haven't read his book yet, but I knew others who did and we discussed a lot about it.