I Wasn't Prepared for My Teenage Niece's Mood Swings

So here's the thing. I have this niece, let's call her Lily. She just turned sixteen and we're close enough that she talks to me about a lot of stuff, but not as close as one might think given our age gap. Anyway, last weekend my sister was out of town so Lily came over to stay with us for the week while her dad took care of work stuff. Everything seemed fine until Wednesday morning. I woke up early and headed straight into the kitchen to start making breakfast. I'm a bit old school like that, eggs and bacon in the morning. But as I crack an egg, Lily storms into the room with this look on her face that could freeze over a lake faster than you can say 'winter'. 'Why are we having bacon again?' she demands. I tried to be understanding, explained it was my routine and no one else seemed to mind eating it. She just rolled her eyes and grumbled something about how I'm always so traditional. Well, that didn't sit right with me at all. So when I went upstairs later in the day to ask if she wanted a snack, she snaps back sarcastically about how she doesn't eat 'junk' like chips or cookies. I felt this wave of defensiveness and said, 'Well maybe you should try something different from your normal routine.' She just gave me this look that was somewhere between annoyed and sad. The next day she left for a friend's house without saying goodbye and didn't text back when I tried to reach out. Now I'm here thinking about how I really upset her and wondering if it was worth getting into an argument over something so small.