Lets start with some background. Way back when, when I was young and still had my license I played paintball, and if you've never played they build paintball fields out in the middle of nowhere. On the way there was this long, windy road that was well paved and a lot of cyclist would ride it every morning around the time I was on my way to the field. Unfortunately, there was only the road, there was no sidewalk and no bike lane. So, the cyclist would have to ride on the side of the road and would hug the line of the road to let cars go by.
I don't know if this is the cause of rage of most drivers, but at the time, I would attribute my annoyance to two reasons:
1. My safety, this was a winding road, in Florida, and you always have to vigilant because the drivers are morons, so when I had to move over to avoid the cyclists, if I was on a curve I wouldn't be able to see oncoming cars.
2. I didn't understand why the cyclist would put themselves in that situation. I'd see these guys and wonder why they would consistently put their lives in my hands. Courtesy is hard wired into my mind(my parents raised me right, despite my attempts to thwart them when I was young and dumb ) and I would slow down or move around, try and keep them safe, but it got annoying because I was thinking "suppose I was a new driver and wasn't paying attention? All it takes is a text message(I hate people who can't get off their phone when driving) to send me flying into a group of bikers with nothing to protect them but a bowl of Styrofoam and form fitting pants."
So, my opinion of cyclists were these morons with a death wish or too much faith in their fellow man who put the burden of their safety into my hands and it was annoying. It was like those kids who think there is no better activity to do on a jungle gym than to nose dive off the monkey bars. I wanted to put every single biker in a corner to reflect on their actions because I didn't like being put in charge of the lives of those cyclists.
So, now that I am a cyclist myself, I do everything in my power to not put other drivers in that situation and the "share the road" T-shirts kind of annoy me becomes it comes off as pretentious: "I'm on a bike and don't feel like paying attention, so you should watch out for me because I am not going to watch out for you."
So, to summarize, I think some of the rage from drivers comes from the cyclists simply not paying attention and turning their point A to point B commute into a sick game of "dodge the moron." So, to everyone who has read this far, you're as responsible for those near misses as the "*******" is driving the car, so try and give the 2 ton metal object the right-of-way at all times.