Motorists prefer not to have cyclists on the road with them, because cyclists interrupt the vegetative state in which so many people drive.
Passing a cyclist demands too much attention. It interrupts the uniform flow of uniformly-sized objects in fluid motion like mindless particles in a scientific demonstration.
The motorist trance happens by itself as a result of the repetitive, boring nature of the activity and the system set up to direct it. It is not a character flaw. In fact, it is a major contributing factor in collisions between motor vehicles. Drivers are simply "in the zone" and space out because that's what curious creatures like humans do when faced with a stressful but tedious environment. Driving, even without cyclists on the road, demands attention, but it demands the same attention over and over. It presents multiple variables, but repeats so many of them from a limited list that the driver is ill-prepared for ones that come up less frequently, like emergency vehicles or accidents. A visual target as small as a cyclist really gets lost in that mental clutter.
Small wonder that motorists seek distractions, such as a telephone conversation with a friend or business associate. Or maybe they're looking a a map or GPS readout, listening to talk radio or drifting off into a pleasant memory. What if the driver suddenly wonders whether that thing they definitely need today is not in that pile of stuff they dumped in the passengers' seat first thing this morning? What if the kids start acting up?
Those who like pat answers will now start to snap them out, if they haven't already.
"Get off the road!"
"Death to motorists!"
Most motorists are aware of cyclists in a general sense. The problem is keeping cyclists in each driver's mind, minute by minute, mile by mile. In a good way.
If you want to make headway with the motoring public, you have to show them what's in it for them. Your own rights as a two-wheeled weirdo come a distant second. It isn't right, but it's how the human mind works. Most of them don't want to commit vehicular manslaughter (though some undeniably do) but many of them wonder why they have to slow down to wait behind us or deviate from their path. How do we fit into the whole ecosystem? Why do we merit protection as an endangered species?
We have to sell ourselves.
Two arguments carry some weight: 1) A cyclist does not tie up traffic as much as another car in an overloaded system. 2) A cyclist does not take up a parking space some desperate motorist needs in order to ditch the barge and proceed on foot. In an area where space can be wasted, such as a big box store or a sprawling shopping mall complex in what used to be food-producing agricultural land or pleasant countryside, parking lots can be vast, only filling up (or nearly so) during the Christmas consuming frenzy. But in an urban setting the parking argument has some real weight, as does the one about congestion. But motorists need to be reminded. Here is a good topic for targeted generic advertising.
On the flip side, bicyclists could use secure, covered parking. That takes up space, but not as much as a parking garage for cars and SUVs.
In suburbia, small towns and more dispersed developed areas, both the congestion and parking angles fall far short. But in those areas there is often enough elbow room in the public right-of-way to allow for some infrastructure tweaks to reduce friction between user groups.
It's true, motorists should respect the rights of all travelers, be they cyclists in the lane with them or pedestrians on the sidewalks and in the crosswalks. But first you have to break their trance.