Idiots in their mirrors and cell phones.
I don't mind getting a new car, but getting sandwiched on my bike is less appealing. And people give me dirty looks when I position my front wheel like I'm trying to split lanes.
People that merge, turn right, turn left, go straight without looking at the traffic light and for existence of other traffic suck too, especially on a 45+ MPH road, hard to maneuver on standing water, ice, other fun stuff.
This post will be too long, to sum it up, I hate everyone. Everybody sucks in own their special way.