Why do the men in the platoon shake the dead man's hand?

Just after he joins the Alpha Company, O’Brien is confronted with one of the first dead bodies he has ever seen in his life. Enemy fire hits the company and Cross orders an air strike on the Vietnamese village. The soldiers watch the village burn. They enter the town, which is empty except for a few dead bodies.

The soldiers joke around with one dead man. They shake his hand and say hello and encourage O’Brien to say hello. He becomes a company mascot of sorts. The soldiers sit him up at meals with them and pretend to feed him. O’Brien finds the humor grotesque and disturbing and retreats to his tent.