You know that rush of adrenaline you get when you narrowly miss being hit or hitting something? All those little annoyances when you see others doing things like running red lights and tailgating?Reproduced here with Yolanda's kind permission.
On your bicycle you use physical exercise to burn off that adrenaline as soon as it's produced, just like nature intended. In the car, however, it stays in your system. Every subsequent burst increases your levels. Adrenaline left unchecked results in either rage or panic and of course rage is the choice for power. We respect a controlled rage but not anxiety and fear. So they rage, gradually, they rage more and more. At some point they forget the base point, the point of reference and don't realize they're not being assertive, they're being asses.
Worse yet, most folks are oblivious of how much chemistry directs them. They don't believe or accept that their moods aren't justified simply because they're feeling that mood. It's like "well if I feel this way, it's natural because I wouldn't feel this way if I didn't have a good reason." Never willing to accept that they could be in the grip of a passion with zero basis beyond buildup of hormones like adrenaline.