What motivates them? Packed together like sardines with the distinct possibility of a catastrophic crash and little hope of breaking free, I can't understand the desire to keep going mile after mile, day after day.
Okay, I can see the thrill of the few that breakaway for an attack and maintain it long enough to be noticed; the hope that they will possibly hold their lead to the finish.
Maintaining enough space between riders to keep from crashing would seem to require an immense amount of concentration. As a newbie group rider it scares me to get within a wheel's diameter of the person in front although I do overlap at times. I notice they do too. Of the close to two hundred participants in the Tour de France only a relatively small percentage even get noticed. So in effect, I guess they are racing against themselves for the satisfaction of having done it. And, I suppose it is a training exercise for possibly eventually placing near the front.
Still, seems like a thankless job and an awful lot of pain. Guess they just love to ride.