The Spanish expression cómo no (literally "how not") is similar to the English "why not": It's a rhetorical question used to add emphasis to an affirmation. You can find an excellent example in the newest episode of our Argentinian telenovela Yago:
¡Sí, cómo no, suban, suban!
Yes, of course, get in, get in!
Caption 49, Yago - 7 EncuentrosPlay Caption
You must remember that, even when the phrase means "yes" or "of course," formally, it's still a question, so it's very important to place the orthographic accent on the word cómo, as you can see in the following examples:
¿Puedo pasar? -Cómo no, adelante.
May I come in? -Of course, go ahead.
¿Me das una mano? -Claro, cómo no.
Can you give me a hand? -Yes, of course.
Yo le dije: Cómo no, esperamos verlo pronto.
I said to him: Of course, we hope to see you soon.
Be careful, though. The actual question cómo no also exists in Spanish. In the following examples the interrogative word cómo (how) appears before the word no (no), not as an affirmation, but as an indirect question with a negation:
Ay, Dios, ¡cómo no voy a estar tenso con el estúpido de Lucio!
Oh, God, how am I not going to be tense with that stupid guy, Lucio!
Captions 31-32, Yago - 2 El pumaPlay Caption
It can also be used in a direct question:
¿Cómo no vas a saber? ¡Yo te avisé!
How come you don't know? I warned you!
Without an orthographical accent, como no can also occur in Spanish, usually as part of a conditional. It translates as "because" or "since":
Pero como no se venden, habéis recurrido al sistema de la permuta.
But because they aren't selling you, have resorted to the swap system.
Caption 42, 75 minutos - Gangas para ricosPlay Caption
Pero como no vine a corregirte, sino a avisarte que...
But since I didn't come to correct you, but rather to advise you that...
Captions 15-16, Muñeca Brava - 7 El poemaPlay Caption
Thanks for reading.