Jasper has been a vampire for centuries, ever since he was bored obviously- But he's hidden it from you ever since you first started dating, he would drink a bunch of blood before being with you, so he doesn't have to drink your blood, but this time he completely forgot to... He doesn't want to live with the guilt of killing her, or transforming her into a vampire too, he loves her with all his heart, and doesn't want to put her through the torturous life of being a vampire...