difficult quiz item, differentiating purpose from originating cause

Question

I just missed this question:¿Viajan ustedes ________ placer? Are you travelling for pleasure?I chose "para" because I read "for pleasure" as indicating purpose. The endorsed answer is "por"; apparently I'm supposed to read "for pleasure" as an originating cause.Looking at the examples here, the most relevant would seem to be:Va a estudiar medicina por seguir la tradición familiar.She is studying medicine because it's family tradition.It seems to me, you could also use para here, with a different meaning:Va a estudiar medicina para seguir la tradición familiar.She is studying medicine to follow the family tradition.Here it's pretty easy to tell them apart because they differ in English. WithAre you travelling for pleasure?... I guess I don't even know what "originating cause" means in this example. It's because you wanted to obtain pleasure, right? That seems to me like a purpose.If the originating cause were a noun ("amor", "los niños"), or a pre-existing state/action ("ser el primo"), then it's pretty easy to see it's an originating cause as opposed to a purpose. But for a verb that seems to relate to future action, I don't know how I would know.Any guidance?

Accepted Answer

Hola David
Yes, I think that's a good tip. Nevertheless it's true that sometimes those specific uses of por and para get a bit mixed up, because you can also have this:

Le dije que sí por no pelearme con él.
vs
Le dije que sí para no pelearme con él.

You could translate both these sentences with "so that":
I agreed so that I wouldn't have an argument with him.
This would sound as a finality (in order not to get into an argument with him)
But the first one using "por" can also have that extra nuance of "I agreed "because" I didn't want to get into an argument with him. (adding the "cause" element)
So, sometimes there is a very fine line. 
Saludos
Inma

Answer

I see now I wasn't completely tracking that placer is a noun, not a 
verb. And I see in the examples for using para for purpose there are 
none that have the purpose/goal/objective expressed as a noun. It's 
always an infinitive.So now I'm thinking when there's a case that
 seems ambiguous to me (as to whether we are talking about purpose or 
"originating cause"), if we have a noun, it'll probably need "por".