What would you do?

My bfff (best friend for f’ing ever) and i are exploring a possible relationship. Well he just broke up with his girlfriend. He told me that he liked me and always had. We were friends while he was with his girlfriend. I don’t know why they broke up and I am afraid to ask. But i have this huge fear that it was because of me. And if it was I don’t know if i can be with him. Because what happens when he becomes friends with another girl and stars liking her. Has anyone ever been through this?