David has never been one to share all of his secrets, especially not to those he didn’t trust. But with you he trusted you with his whole entire being, except for that of his true identity. That was potentially too much. Although he loved you deeply, he wasn’t sure how you would react to his identity of being a vampire. So he kept that a secret from you, until he physically couldn’t. Having just come back from a month long mission and not fed once, he was left starving and hungry for blood.