The function of こそ

[kuritiusu]

What is the meaning of こそ in a sentence like this:
それでもこの出来事が君を苦しめるだろう だからこそさよならなんだ。

[Mike Cash]

Short answer: it provides emphasis to the proceeding element. In this case, だから.

In other words, the difference between something like "That is why...." and "That is precisely why....", for example.