Why do I feel this way?

I’m in a relationship with a loving boyfriend. He’s so understanding, loving, nice, sweet, caring, open-minded, all the loving words there is! We are going to hit 9 months this coming 22nd. Everything was always against us, my parents and my sisters. However, my sisters don’t care anymore. We are both 16. Still young. However, I feel sad or sometimes even upset. I don’t know why I feel that way. Is it just mood- swings? I’m not unhappy in my relationship at all.