Kyle Broflovski thought of himself as Stan Marsh's training bike.
It wasn't because he was being ridden, because remarkably, he wasn't. No, it was because he was Stan's training bike. Everyone starts out riding a tricycle, and then they get on a training bike, which is substantially different, because the wheels come off. And when the rider has worked up enough courage to take the wheels off, one of two things happens: Either he falls off and never wants to get near a bike again, or he wants a bigger, better, nicer bike.
If you fall off your first two-wheeler, maybe you try a few more times — get back on, attempt to keep steady. But ultimately you're wounded, and the memory of falling off in front of everyone — all of your friends — is painful. And so you shun bikes altogether, sticking with rollerblades. You hang your childhood bike up in the garage, where you see it sometimes, and it reminds you of your failure. It's a warm, bittersweet memory: You tried, you really tried. But you just aren't a person who can ride a bike.
Infinitely more horrifying for Kyle was the second scenario: If Stan could ride a two-wheeler, if he was happy riding a two-wheeler, he would want a better one. He would sell his old bike to buy a new one, and ride that. He'd love it. He'd ride through town — or a college campus, or a big city — with the wind whipping through his hair, racing toward a destination that his old bike would never see. Even in this scenario, in which Kyle imagined Stan sometimes using rollerblades, he'd still sold his training bike.
But whatever happened, Kyle knew that Stan would never want to ride the same lame training bike forever. And so he dreaded the day when the wheels would come off, because that would be the beginning of the end.