Is it normal to be scared to date your best guy friend?

ok, so i have known my best guy friend for 15 years now since we were two years old. We are now 17 and still both of Our families say we are going to date and one day get married. He has always been there for me every time i have dated and gotten my heart broken. My recent break-up, he texted me and said he will never leave me. i had him promise. He is the only guy i will ever trust with my whole life. Every time i date someone, we hardly ever talk and never hang out. This break up was different though. We now talk everyday, and always talk in school. I know he likes me, he knows i like him. We're always saying i love you to each other, playing around. I have a big fear of dating and breaking up and losing a 15 year friendship. I know he would never hurt me but I'm still scared. someone, please help with this! i know I'm young but there is something telling me to take a chance but something telling me wait till we're older, but, i don't want to miss my chance. p.s he is always saying we're gonna get married one day, and i have always, always been my complete self around him.