When someone tells you what they think one of their character flaws is, you should pay attention to that. They’re not telling you that because they want you to contradict them; they’re telling you as a warning. All warnings should be given proper heed and attention.
I’m not suggesting that you encourage or ignore the bad behavior. As an example, if someone you are just getting to know says, “I’m not a very good friend,” this isn’t your opportunity to prove you have some superpower that changes someone’s personality or behavior. And even if you try to be the best friend ever to the self-proclaimed bad friend, that doesn’t mean that person is going to change that attribute of theirs to compensate and prove otherwise. No, this is your opportunity to learn why that person stated that, then decide if it’s a risk you want to accept into your life.
This is tough to do, especially at the beginning of a friendship or relationship. At that stage, we’re all a bit giddy and stupid, or giddily stupid, to the point of actually behaving stupidly. Think about the times you’ve seen friends do stupid things in a relationship because they’re blinded by the person’s charms, rather than being realistic about the person’s flaws. It’s not to say you can’t still pursue a relationship with that person, but you have been forewarned.
Likewise, how does that person respond to you when you tell them what your greatest flaw is?