Por qué, por la que? Relative pronoun issue?
 

I was writing a paper last semester for my class making up a legend and I normally write "the reason why" as "la razón porque". My friend corrected me and she wrote "La razón por la que" which I never used it like that but it looks like relative pronouns which I never really know how to use. I asked her why and like usual, people tell me they grow up knowing what is correct because it is their first language but they cannot explain why it is used this way. So, why do I need to write it this way?

It translates to me, "The reason, for that,.." which gets the same point across. I know I shouldn't directly translate

excerpt from the story I had made up:

Toda la gente supo de Gloria y la consideraban la mujer más gorda en el país y todos comenzaron a llamarla “Gloria la Gordita”, ella vivió sola por el resto de su vida comiendo la misma cosa siempre y esa es la razón por la que hoy en día en México, llamamos a esa comida “Gordita”.

In Spanish, explanations are always expressed with an explicit antecedent.

"La razón porque" is not Spanish. "Porque" is a conjunction, while "la razón" is a noun, and they don't go together.

- No bebí el café porque estaba frío. -> "Porque" is explaining my action of not drinking the coffee.
- Los niños están contentos porque están de vacaciones. -> "Porque" is making the link between happy children and their reason for being happy.



But if you want to make an explicit sentence with "la razón", "la causa", "el motivo", and similar nouns, then your article is also explicit.

- La razón por la que vivimos aquí es que nos gusta el campo.
- No te diré los motivos por los que me divorcié de Juan.
- ¿Quién nos puede explicar las causas por las que la gramática es tan complicada?