How come the subject is omitted in Spanish?

Question

You can find hundreds of sources where they say that the subject can be dropped if it doesn't add any additional information. As "voy" is the 1st person singular conjugation of "ir", you know that the subject is "yo". That sounds reasonable when coming from the English language, where conjugation is basically missing.

Well, but my mother-tongue is German and in German we do decline verbs as well. As "gehe" is the 1st person singular conjugation of "gehen", the subject is "ich". But in German you generally don't omit the subject.
And that's true for other languages, too. French, for instance. As "vais" is the 1st person singular conjugation of "aller", the subject is "je".

So, how come that Spanish language has developed in a different way?

Answer 1

Lovely question!

The conjugation of a verb in Spanish has more info about the person: in German sie, Sie und wir are conjugated the same way, and here is the key: they always coincide. Also in the singular, the verb for some personal pronouns very often coincide (ich muss, er muss). So you do need a personal pronoun in German. And well, in English, as you say, for obvious reasons you cannot drop it.

As for French, you do write different, but phonetically there is no distinction between (tu) vas and il va (unless you have some vowel right after). This holds not only for aller.

So, basically, my hypothesis is that since in Spanish from Spain in most of the verbs in various tenses the conjugation fully determines the person, there is no need to keep track of it. Then, although in some versions of Latinamerican Spanish ustedes is used instead of vosotros and the former has the same conjugation of ellos, dropping the pronoun was inherited.

You basically economize without loosing information. There is no information redundancy. (Nice feature, isn't it? [1]) To support the statement, there is (at least) another language which, to some extent, does the same: Russian. There conjugation is so irregular, that you might omit the pronoun. (Example: знать in present is conjugated as знаю, знаешь, знает, знаем, знаете, знают. The forms are all different)

However, you might want to keep the personal pronoun if you want to emphasize the person. Dropping the person would be like in Dutch choosing je as a personal pronoun and saying the pronoun would be choosing jij.

Late Edit: I just realized there is a name for those languages: pro-drop.

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[1] I think the German analogous of "no information redundancy" –although departing from verbs and I'm unsure if that was how it developed– would be that in the schwache Deklination of adjectives you kind of drop lots of their endings, for the determinate article already has them:

ein netter Kumpel Vs. der nette Kumpel (← you no longer need to write netter, for der is already saying that the noun is masculine, while ein doesn't)

Answer 2

As far as I know Latin also allow to drop the subject: v.g. cogito ergo sum does not have an explicit subject ego.

So the question might be backwards: why other Latin-derived languages such as French require the subject? In Italian, Catalan or Galician you can drop it. In Portuguese you cannot (yet Portuguese and Galician are almost the same language.)

As Spanish has verbal declination by person, it reduces ambiguity compared to English, but that's only part of the answer. Spanish just evolved that way while English had a different evolutive path.

French had added ambiguity in verb declinations given its phonetics, and this is partly solved by forcing the subject's pronoun, but some linguists actually see that French “subject pronouns” are really part of the declination. Compare:

Je vais au cinéma.
Moi, Je vais au cinéma.

With Spanish:

Voy al cine.
Yo voy al cine.

The actual explicit subject in spoken French is moi, while je vais is the first person declination, although in writing it still behaves as je being the subject and vais being the declination. (and moi being just an emphatic repetition.)

This phenomenon is replicated in some Francoitalic dialects such as Piedmontese, but it does not explain why Galician keeps the dropping of subjects, while Portuguese does not.

Which leads to the actual answer: that's how each language is.