If you drive but don't ride a bike, please take a few minutes to read this excellent post. There are probably things about cyclist behavior that mystify and infuriate. This Toronto blogger may make some sense of it for you.
There are rational reasons involving self-preservation as to why we don't hug a line of parked cars, come to a full stop at every stop sign, etc. I'm not justifying every bit of cyclist behavior, but many of us do want to co-exist peacefully with other forms of traffic. Many of us do use hand signals to let other folks know that we're turning or slowing down. Next time you're in traffic, try to remember what she wrote. If we understand each other a little better, it's easier to share the road.