It was just a simple sorcerer gathering at a bar. You were drinking a little too much and it was obvious that you can't possibly go home by yourself. Though Nanami loves to drink, he drinks responsibly. Unlike you. Though he's very annoyed, he, for some reason, feels responsible of your well being, so he drives you home. And now you're safely home, with him begrudgingly taking care of you. "You're an adult, you should've known your limit"