John is mesmerized by a beautiful car, truck, SUV, motorcycle, airplane, helicopter, hell he's mesmerized by a clean, shiny TARDIS so long as it's something that'd get him from point A to point B. That's his manly thing, he likes transportation devises. He likes their build, frame, shape, size, color, and he likes to be in control of them.
Sherlock, on the other hand, hates them. He'd rather walk than ride in or on anything. It's not that he doesn't like to be in control. Oh god, he loves being in control of something that large or powerful. It's that he's -is it even true?- afraid of them. At any moment they could break or hit something else or generally just hurt you.
And Sherlock isn't often afraid of pain. No, things like that he can suppress. Often times he's been held captive, or injured, or in any general danger. It's that in a car or train or on a motorcycle, it's yours or another person's fault. Sherlock actually couldn't forgive himself for a long time if he put any other person in danger because of his poor driving skills, and he definitely would never forgive a pilot for putting him into a plain crash.
The fear of vehicles comes from when Sherlock was a kid. One of his earliest memories, he was three or four, was a time when his father took him to town to go to the zoo. Sherlock was so excited he couldn't sleep that night, he couldn't eat in the morning, and he could not stop bouncing off the walls in sheer excitement.
When Sherlock and his father got out to the cars, his father let his driver have the day off so it could be just him and Sherlock. They got in the car and went to town and everything was fine, until Sherlock was being so rowdy and wild in the backseat, his father was watching him in the rearview mirror while laughing hysterically.
What happened next, Sherlock doesn't remember. What he does remember is waking up some time later in a hospital being checked for further injuries than a few cuts and bruises and his throbbing headache. His father was all right, he had a large gash on his forehead from where he hit the windshield and his arm was bandaged from being pressed against his door.
From then on, Sherlock hasn't trusted any sort of transportation vehicle, even if he is in control of it.