I don’t understand anything

I’m 16 and have been in a relationship with a girl for a year. She’s good looking, and intelligent. The problem is, she doesn’t want to have sex. I’ve been begging her for months but she always says no. I don’t get why, she’s always talking about how much she loves me and how she enjoys being with me. But when I bring up sex, she’s like “I love you too but that’s something I don’t feel comfortable with.” Why?