La penso così

[toscairn]

I learned from my dictionary that "lo" is usually used when we refer to what is previously said or written. For example: Io lo so. With that in my mind however, I don't understand why the bold "la" in the dialog below is used instead of "lo." If "la" is used, does it refer to "una vera pizza" or the whole idea of what Rose said about how to eat authentic pizzas during the afternoon?

Grete: Volevo mangiare una vera pizza.
Rose: Ma a Roma le pizzerie aprono solo la sera. Secondo me non e' possibile trovare una vera pizza nel pomeriggio a meno che non si vada a Napoli.
Grete: Anch'io la pensavo cosi'.

Or is it so that the bold "la" in the above text refers to "questa cosa", which is not explicitly said in the dialog?

One more question, per favore, why is it that "la penso cosi'" is far more common than "lo penso cosi'?"

 

[Leti]

Maybe someone has already answered you. It's very simple: "la" is sometimes used in phrases such as "la penso cosi'", "ma la vedo brutta", "me la passo male" etc. to mean nothing very precise. In your case, the meaning is: that's my opinion too. "Lo penso cosi'" is not correct at all, nobody will say it: you may say: "lo penso anch'io", and you intend something very precise someone has said; in "la vedo cosi'", "la" has a very general meaning, and with "cosi'" you refer to what has been said.
I don't know whether it's clear: it's an idiom, that's all, and there are other similar usages of "la".
by

 

[toscairn]

Grazie mille!