I think most of it just frustration at being held up, especially on busy country roads where you have a constant stream of oncoming traffic and very few safe overtaking places, makes you cringe when you see people get so close overtaking a cyclist.
That said I think it takes either bravery or perhaps stubbornness to cycle at rush hour, I tried it and you could feel the hate from the drivers behind
Combine the above with muppets overtaking too close and the potential to be wiped out on any blind corner (you can't drive expecting to meet a cyclist on every blind bend, the fact more don't get hit amazes me) means I'd rather stay in my safe metal box getting fat.
That said I think it takes either bravery or perhaps stubbornness to cycle at rush hour, I tried it and you could feel the hate from the drivers behind

Combine the above with muppets overtaking too close and the potential to be wiped out on any blind corner (you can't drive expecting to meet a cyclist on every blind bend, the fact more don't get hit amazes me) means I'd rather stay in my safe metal box getting fat.