First of all figure out the story behind it then you'll be able to tell why this happened.
Man1: Gets off train and walks down a dark alley
Man2: Knocks out Man 1, cuts off his arm wraps it up and sends it off to Man 3
Man3: Opens parcel looks at the arm and smiles then sends it off to Man 4
Man4: Opens parcel looks at the arm and smiles then sends it off to Man 5
Man5: Opens parcel looks at the arm and smiles then sends it off to Man 6
Man6: Opens parcel looks at the arm and laughs then chucks it in the bin

Why are these men smiling and laughing?

They are having a bit of 'armless fun?

Come on, I am right aren't I? What's my prize?

Five men were lost in a snowstorm without any food. In order to survive they decide to eat each other's arms. After four arms were eaten they were rescued. The last man promises that he will cut off his arm and send it to the others to be fair.

He gets off the train and walks down a dark alley and finds a homeless man. He knocks him out, cuts off the homeless man's arm and sends it the the first person. etc.

He has a disarming personality?