It's very frustrating when a guy you're blowing in a bookstore or theater pulls away to finish himself off, maybe thinking it's impolite to cum in your mouth. Of course, that was what you were hoping for.
Car head is exciting because that won't happen. No guy wants to shoot cum all over his upholstery so there's an implicit contract that he will cum in your mouth and you will swallow. I love it.
I agree. It is especially frustrating if they've made you work for it for a long time. When I spend a lot of time working a guy's cock, I'm really looking forward to my creamy reward. Mmmm.