I've seen this meme circulating lately, pointing out one of the many valid reasons to learn to speak dead languages properly:
My first thought was, "this is why we need to mark long vowels!" But I realized, to my shame, I don't know how to express this extremely important sentiment in Latin.
What is the idiomatic equivalent of the English "this is why…"? All I can think of is a roundabout construction, "it is because of this reason that…", and that just feels awkward.
Where English uses relative constructions such as 'It is for this reason that...', 'This is (the reason) why...', and so on, to add emphasis, Latin tends to dispense with the relative clause and rely on word placement instead.*
Therefore, I would just use a word like propterea – or a phrase such as hac(ce) ex causa, hanc ob causam, or the like –, emphatically placed at the beginning of the sentence. In initial position, all of these could, by themselves, have the force of, 'It's for this reason that....' or 'This is (the reason) why....'
* See, for example J. Mountford, 'Bradley's Arnold Latin prose composition §82 (cf. §156):
An emphatic order of words often expresses in Latin what English can express only by a relative clause following a main sentence which begins with the impersonal it.
Agricolam laudat iuris legumque peritus. It is the farmer whom the lawyer praises.
The first word that comes to my mind is quapropter. Lewis & Short gives it the clunky translation of "wherefore" or "on which account," but it can be much less stuffy than that. Here's a good example from Terence:
ridiculumst istuc me admonere, Clitipho,
quasi ĭstic minor mea res agatur quam tua.
hic siquid nobis forte advorsi evenerit,
tibi erunt parata verba, huic homini verbera:
quapropter haec res ne utiquam neglectust mihi. (Clitophon, Act II)
My (loose) translation:
It's ridiculous for you to tell me this, Clitiphon,
as if this didn't concern both of us!
If anything happened to go wrong for us,
You'll get words--I'll get wounds!
This is why I won't ever be negligent in this matter.
I wouldn't disagree with the answers of either brianpck or cnread, but what is wrong with hoc est, cur …? It's simple, short, easily understood and, if you google the three words, you will find plenty of attestations.
Quare can be translated as "This is why"
in https://en.wiktionary.org/wiki/quare#Latin, they translated:
"Inveniuntur enim praeter amnem mirae villae et horti, qui a regibus Franciae in XVI° saeculo structi sunt: quare Liger hodie saepe regale flumen vocatur"
as "However, besides rivers/streams are found marvelous estates and gardens, which were constructed by kings of France during the 16th century: which is why the Loire today is often called a royal river"
