You can say "こそ" as an emphasis. ex: this book こそ (enphasized) what I'm looking for. I finally found it! this book is what I'm looking for. (simple sentence). It should be that there are many stuff and not so easy to find or decide something, but you found the one (decided the thing) you want, you'll say "こそ". There is another example: A: thank you very much B: same here When you are the person B, you can say "こちらこそ". In this situation, you should say thanks to the person A before he saying it, but it's too late. Hence, you can say "こそ" because "Same here" is not enough to express your gratitude. So it's going to be the same thing as an above example. For showing how much you are appreciate it, you'll be emphasized by saying こちら"こそ".

こそ　英語で~ only（one) 、 the very が使わています 単語なら　qualified 準じると思います　　限定示す副詞　名詞あるいは動詞のば型＋こそ～で限定を示す言葉です　　例としては　　君こそ、ふさわしい　すればこそ、価値がある　　することこそ、かち価値がある。