Te la

Te la
 

¿Por qué te la agarras con el pobre de Roberto?

Why do you take it out on poor Robert?

The English translation has you as the subject.

"it" frustrations ? why "la"

This looks like .. you grab (agarras) her(la) for/to yourself(te) with Robert.

How does this work?

I searched "agarrarsela", and found some entries at different on-line dictionaries that offer 2 different slang meanings for this expression:

1. Contract a venereal disease
2. Get drunk

Based on this, I suspect that the intended meaning of "¿Por qué te la agarras con el pobre de Roberto?" may be "Why do you get drunk with poor Robert?".

Thanks, yes getting drunk is one meaning but it is also shown as grasp, seize, grip. I provided the given interpretation which is "..take it out on.."

Perhaps it means that she is grasping hold of an idea (la) for your own ends (te) to the detrement of Robert.

Is that possible?

I don't think so, because there is no explicit feminine singular noun either within the same sentence or within the context of the sentence that can function as the referent of the pronoun "la" and that also has the meaning "a/the idea".

Without such a referent, "la" and its associated verb usually are part of some idiomatic expression that has an arbitrary, conventional meaning. To my ear, that is the case with your original sentence and is why I think an appropriate translation is "Why do you get drunk/are you getting drunk with poor Robert?".

What AdA said
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"To take it out on Roberto" is the right meaning, where "la" is as indefinite as "it" is in English. It can be anything; for this case, probably "la bronca"/"la pelea"/"la mala fe", but it's not necessary to know precisely what "it" means, since the expression has a meaning by itself.

A more common way to say it is "agarrarla con alguien" (without the pronominal), but it doesn't make much difference.

Ah!

So "agarrar(se)la con alguien" = "take it out on someone", while "agarrarsela" (without "con alguien") = "get drunk/catch a disease" depending on context...

I didn't know the "catch a disease" meaning. But you are right. If it means "to get drunk", then "con + alguien" is not used:

- Estoy crudo. Nunca la había agarrado como anoche.
I have a hangover. I had never been as drunk as last night.

- Juan todavía está dormido. La agarró ayer todo el día.
Juan is still asleep. He was drinking yesterday all day long.

- Vamos a agarrar una de aquéllas con este tequila. (Notice "con" is the substance that will make us drunk.)
Let's get epic drunk with this tequila.

- Roberto y Jorge la agarraron desde temprano. (To avoid the ambiguity, we prefer to picture them "agarrándola" together.) ;)
Roberto and Jorge started drinking early.

"la" is part of the verb here. It's what is called a lexicalized pronoun, like "la" in pasarla bien.

no te agarres conmigo (don't fight me, don't pick a quarrel with me)
no te la agarres conmigo (don't take it out on me; don't vent on me the angry feelings you develop elsewhere; don't take any minimal objection you may have regarding my doings as an excuse to unload all your repressed anger)

"la" is sort of an electric charge which is looking for a kind of lightning rod.

Thanks Angelica,
I figured the "la" part it was more how the agarras and the te that was a little troubling. The agarras already says "you" D.O. but te in this position indirect you, suggesting you grasp it for yourself (for your own motives perhaps).

There is no difference between using "te" and not. It's just a matter of regional differences and personal preferences when talking. :)