からこそ means 'precisely because.' It is stronger than just から. It emphasizes that the reason is the very thing that makes the result true. You use it when you want to stress cause and effect, often in explanations, justifications, or emotional statements. For example, if you say 'because you are here, I am happy,' using からこそ adds the nuance that your being here is the only reason, and without it, I wouldn't be happy. It is common in both speech and writing, but it sounds a bit assertive and is more frequent in formal or emphatic contexts. In casual conversation, you might hear it, but it can sound dramatic. Note that からこそ often pairs with のだ or んだ at the end of the sentence, creating a pattern like 'からこそ...のだ.' Also, when giving a negative reason, you can use ないからこそ to mean 'precisely because it is not...'.