I've always assumed it was ego, hubris or just plain testosterone poisoned machismo.
I weight >220lbs, I ride a large steel frame bike with loaded panniers that stick out like little parachutes.
But after lurking the IHPVA "trikes" forum for years I learned that the only way I'd keep cycling for years would be to maintain an 85~95 RPM cadence so as not to destroy my knees. It works great too.
With all that inertia to waste in starts and stops I've learned to maintain a consistent pace.
From my years of driving large tractor trailers I learned more of how to interpret what traffic is going to do before it happens.
I have a disturbing ability to intuit and memorize traffic light patterns.
What this has all added up to is that I am the evil bastard that can cruise up to a red light without ever slowing down because I know when it turn; often I will slow my pace almost imperceptibly synchronizing with a "green wave" several blocks away.
I can see who is going to try to right hook me and just pass them on the left.
I plan far enough ahead to realize that time saved on a MUP is lost at crossings where MUP traffic has no priority or ability to cross safely.
Because of all that when I see cyclists that speed up to pass me and other cyclists just to slow down I just feel sorry for them that they have so little self worth that they must remind themselves every few minutes that they are better than all of the rest of us because they are faster.
Both Sides of the Coin
In every passionate disagreement there is a kernel of truth that the opposing side cannot refute.
The illusion that keeps us apart is that these opposing truths are different and implacable; they are different sides of the same coin and to deny the other side is to deny your own.
A coin, like life, cannot exist with only one side.