Why do I have trouble being direct with my boss about problems?

I have a really weird thing with my boss that I just cannot be smooth or direct with him and bumble like an idiot when I’m explaining something to him or asking a serious question. I’m a mid level manager and have no issue at all talking, being direct, or explaining issues with anyone. My employees, same level managers, even high corporate bosses I have no issue at all having hard conversations with them. Never have any issues being strong and direct with anyone.. except him. It makes no sense to me because we get along great, he’s a laid back “cool” boss. Not very “by the book” and kind of just goes with the flow. So what is the mental block here? Why do I feel so scared to ask him for things or explain issues that I’m dealing with him?