How many people were nervous about a relationship?

One of my good friends has been asking me out and I always decline. I've been really good friends with him for a few years now. He's the type you marry and he's veryyyy sweet. But I'm scared to even give him a shot. I haven't had a boyfriend in four years but I have dated over the years and they were all scum. I don't know why I'm so scared to give him a shot. Any advice?