The problem is that he is that charming with every woman he meets regardless of whether he is in a relationship or not. Casanova really needs to save the flirting for the woman he’s committed to instead of his coworkers, bank teller, waitress and female best friend. As a result of his irresistible charms, these women are all waiting around for him to ask them out.
As you might imagine, Signore Casanova has a lot of females waiting for him to be single again. These women will probably hate your guts just because you are dating him and will probably categorize your faults (as they see them, anyway) to Signore Casanova at every chance they get. They will even make rude comments about how they don’t know why he’s settling for you. (All the time!) What’s even worse is that he has all those women waiting in the wings when he is single again and rebounds so easily that you wonder if he cared for you at all.