Unfortunately, your cooperation only helps Bob, and Bob's cooperation only helps you.

For a nice rephrasing that makes that clear, see the Chocolate Dilemma:

There is a sack of chocolate and you have two options: either take one piece from the sack to yourself, or take three pieces which will be given to Dylan. Dylan also has two options: one piece for himself or three to you. After you both made your choices independently each goes home with the amount of chocolate he collected.