This is idiomatic German, a very common construction and means "they are" or sometimes "there are". In this case "Es" is a "placeholder" and the verb agrees with the actual subject. Here is an example to demonstrate the difference between the singular and plural forms of this construction:
"Wer ist an der Tür" (Who is at the door?) "Es ist eine junge Frau" OR "Es sind drei junge Frauen" Inverted: "Eine junge Frau ist es" (Sie (she) ist es) or "Drei junge Frauen sind es", ("Sie (they) sind es") (You cannot say "Es ist drei Frauen")
"Es gibt" is always conjugated the same way, because "es" is the subject. "Es sind" is different in meaning to "Es gibt". In this sentence it really means "THEY are personal reasons" but NOT "There are personal reasons" (Es gibt)
This "native English speaker" uses either of these, depending on the context. Once again native English speakers disagree about what native English speakers would say. I moved from Chicago to Boston and struggled to understand these people. Imagine the differences between Chicago and Sydney. Also, just because it "sounds right" doesn't make it proper grammar.
Because in English, when using a copulative ("linking") verb such as "be/is/are" the number (i.e., singular vs. plural) of the subject (der Nominativ) should match the number of the predicate object, and the verb should be conjugated to agree. So, "They are personal reasons" or "It is a personal reason."
I am english and have no problem with the vast majority of people's theories on the thread but my reason for coming on here has still not been answered. Why es and then sind. Surely it makes no sense. Es gibt....fine. sie sind.....fine but es, singular, sind.....plural. so surely it ought to be es ist. Quite evidently there is a reason for es sind otherwise it would be wrong but nobody appears to have explained why. Apologies if i have missed the answer already given on the thread.
It's idiomatic. Es is a "placeolder" at the beginning of the sentence or clause. Here's a good explanation: http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-P/Pron-es.html?lang=en#Anchor-es-11481
Here instead of saying 'Es sind persönliche Gründe', can we not say 'Die Gründe sind persönliche'. Instead of es(if we consider in English- They is basically a personal pronoun, so should it be used for reasons/grounds which is not a person? ). Instead can we not use the definate article Die here?
Mizinamo's explanation is absolutely correct (of course). But there is a slight misunderstanding expressed in Mini359442's question:
"They is basically a personal pronoun, so should it be used for reasons/grounds which is not a person?"
Although they are called personal pronouns, they are not reserved for use with people/persons. They can be used to refer to any noun/object, even abstract ones such as pronouns. (See what I did there?) "They", in particular, is used to refer to multiple objects: it is the plural form of "it", which is clearly used for non-people but is still called a personal pronoun.
Of course, "they" is the plural form of he and/or she as well.
Note that even the singular "he" and "she" can be used for non-people in some cases: boats, ships, and aircraft are commonly referenced as "she". And when I call my cat,
he pretty much ignores me.
Both are possible in sentences, just as "They are..." and "Those are ..." are possible in English -- depending on what came before.
"They are..." would usually refer back to something you had discussed previously, while "Those are..." would be more common for introducing a new topic into discussion.
Why doesn't it make sense? As in the response to "Please tell me why you aren't coming to work tomorrow?" or to make it less awkward, to "Tell me what made you decide not to come to work tomorrow."
I'm thinking one would easily rather respond "It is because of personal reasons", but that a sentence construct doesn't translate well from one language to another is not surprising. As long as it still makes sense and is grammatically correct... Not a native English speaker, pardon me if i'm off.
Usually we (English speakers) would answer "for personal reasons" - "I'm taking the day off for personal reasons." Using "because" requires something more concrete - "I'm taking the day off because my mother is sick."" "I'm taking off because I have personal issues [to attend to.]"
The plural "sind" tells you that "personal reasons" are the true subject of the sentence. A kind of equivalent in english is "there is/are", and how "there are personal reasons" is different from "they are personal reasons" or from what you suggest. (it's just a parallel though, because German has much closer to "there is" with "es gibt")
The sentence you propose would, on top of the verb being aligned with "es" and be singular, also require a preposition. I wanted to say "für" but i'm not 100% on that, and google translate gives me "Es ist aus persönlichen Gründen". So probably they're right.