こそ is a particle that emphasizes the word before it. It's like saying 'this very thing' or 'this and nothing else.' You can use it to stress a noun, the reason (からこそ), the topic (これこそ), or even a time expression. It adds emotional weight, showing strong feeling or conviction.

For example, when someone thanks you and you want to say 'thank YOU,' you'd use こっちこそ (this side, for sure). Or when pointing out the one thing that matters, like 'this is the best,' you say これこそ一番だ. It's common in speech to show enthusiasm or to correct a misunderstanding.

But be careful: こそ is strong. Using it too often can sound pushy or dramatic. It's fine in casual talk and also in formal speeches, but in neutral writing or everyday conversation, it's used sparingly, for emphasis only.