
Driving in car, you might think overtaking is a totally normal thing. It has no implications - the person in front is slower and you want to go faster, so you overtake. Single white line, no oncoming traffic, change lane, step on the gas, get back in lane.
Not so on a bike. On a bike it's an ego disaster if you get passed and fail to pass the person again. The drill is the same, get to the right of the rider, step on the pedals, upshift, step, step, step, pass him. The problem with doing this on the bike is that your anaerobic and high-intensity fuels can run out very fast, and when that does, you realise that the person you overtook is suddenly on your side, overtaking you. But I tell you: it's damn thrilling for that little second you are overtaking. It's like shooting an arrow straight at the ego saying, "I got better cardio/muscle power/aerodynamics/weight" than you!"
Of course, the thrill varies according to what equipment you use. The greatest satisfaction comes when you are totally disadvantaged - fat tyres while your opponent (notice: "opponent") rides with skinny tyres, or perhaps he has an electric motor and you do not.
The problem, as aforementioned, is always keeping the lead indefinitely. Not likely. Once your opponent decides that you aren't gonna keep it, you gotta work even harder to keep it. He hasn't used up his high-intensity fuels, he does not need to travel the extra distance you rode to overtake him, and he might still be fresher than you, with greater peak power at that point of time. The biggest problem, however, likely comes from the thought that once you do not see him, the race is over. It isn't. He's behind, spinning up those legs, while you are in front, likely slowing down the legs - it's the pain, and your motivation to step on it is now behind you... but not for long.
So why do men train? To keep that ego inflated. Armstrong, Contador, Mercx, you, me, that guy on the Giant who passed me after I passed him while staring in my direction, we all train to different degrees, to keep things fast - more accurately, faster.