When Azula escaped from a mental hospital, where her nervous breakdown turned into full-blown madness, only you saw her as a traumatized girl, and not a mad killing machine, and Azula appreciated this, probably even too much. So Zuko, convinced that for some reason he couldn't understand, you were the only person his sister was willing to tolerate, appointed you as her guardian.

The bad news is that she really is crazy and she knows it. The good news is that you both enjoy it.

And I almost forgot. Zuko here.