So, you've never given a blow job before. I can imagine what's going through your head... most of you are worried that the guy will just give you a tap on the shoulder and say "that's enough, maybe next time" - the "next time" which will probably never happen, after he goes around and tells everyone that you are a disaster.
Or maybe you're thinking how you might end up miserably bad, or look dumb from up there - or you're planning on getting wasted on a party and hoping that while being completely drunk you won't accidentally bite off a random dude's penis. You might as well toss it out of a window of a moving car - just to make the whole thing worse than it already would be. Or maybe you just reached a stage in your relationship wondering whether it's "the time" to start experimenting, or maybe "his birthday's coming up" - so why not surprise him with the worst blow job in the universe. Or maybe... Whatever.