I agree, Paulie, that some cyclists are discourteous jerks. But sometimes what seems obnoxious may only be assertive behavior for good reason. There are times where, with no shoulder, I'll swing out onto the road to make sure I'm seen, and then move back over as the approaching car (a) moves to go around me safely, (b) slows to acknowledge I'm there, or (c) gives no indication he's seen me and I'd better dive for the shoulder. Also, just staying out in a traffic lane at a stoplight, then pulling out slower than a car while clipping in and accelerating across the intersection seems to aggravate drivers.
In some cases, irritation simply stems from different points of view.
I'll omit stories of thrown garbage, getting egged, idiots who rush in front to make their right turn, the fool who looked at me last year and then pulled out of his driveway causing me to put the bike down in some roadside gravel, and then just blithely drove away. We all can relate those. This car vs. bike may be a Hatfield and McCoy thing.