What makes someone Mature?

Being able to see things on a multiple scale of gray instead of pure black and whiteness There's no such thing as truthful evilness and truthful righteousness... Except for very extreme cases

I can remember a very simple statement by my English teacher back in high school. "Being mature isn't about never being immature, it's about knowing when to be immature."

That's a wise teacher you used to have, it's definitely true that you don't want to be with someone who doesn't know when to stop taking themselves seriously