If your strategy is to just use dictionary words your password will have little entropy and even less so if you use grammatically correct sentences. If the attacker knows this is your strategy of choosing passwords cracking one is way easier than cracking a password that has the same length but consists of randomly chosen characters.

Your password is only safe because the attacker doesn't know your strategy of choosing the password which forces him to use inefficient methods of cracking it, while there would be a more efficient way if he knew the strategy you used. Which is security by obscurity.