What does ¿Cómo no? mean ?
a: No voy a la fiesta (I'm not going to the party)
b: ¿Cómo no?!! (Why not? / What do you mean you're not going?)
a: ¿Te gustaría ir al cine? (Would you like to go to the cinema?)
b: Cómo no!! (Of course I would)
a: ¿Cómo no te gusta el chocolate? (How come you don't like chocolate?)
b: Oh! It's too sweet for me...
It can also be used as sarcasm.
Arrested person: Soy inocente! (I am innocent)
Police Man: Como no! (Of course you are)
"Como no" can also actually mean "of course". I had heard the phrase a few times before knowing it meant that, and man, how misleading. :p