That person is not in the road to upset you.
That person could be on his way to work.
That person could be on his way home from work.
That person could be on the way to the store.
That person could be trying to improve his health.

Safety: To improve the bicyclist's vantage and safety margin to hazards ahead.
To be more conspicuous and thus less likely to be overlooked and hit by cross traffic ahead.
To alert you, sooner rather then latter, to the need to either slow down or use the adjacent lane to pass safely since the lane is too narrow for the both of you to safely fit in the lane side-by-side.
To avoid glass, dangerous debris, the door zone and other road edge hazards. Traveling at a normal bicycle speed on a sidewalk is often impractical and unsafe and in many areas illegal.
To prepare for a left turn, moving over one lane at a time instead of swerving across the whole road in one move.

Courteous: - To give you more time and distance to prepare to slow or pass safely.
Legal: - Because the bicyclist is subject to the same rules of the road as other drivers. many areas illegal.

Rightful: Because the bicyclist pays taxes which pay for the roads. Less then half the monies for road building and maintenance is generated from Fuel Taxes, the majority comes from Sales and Property Taxes.
Just because some other bicyclist disobey some rules of the road doesn't mean this one has no right to be in the road.

Please remember there is a "Basic Speed Law" which says "An operator of a vehicle upon a highway shall not drive at a greater speed than is reasonable and prudent, having regard for the traffic and for the condition and use of the highway." Not all conditions permit driving at the speed limit.
Please pass bicyclists by changing lanes (traffic lanes on our roads in the area are not wide enough to share).
Please don't honk at bicyclists because they are in the traffic lane. They know you are there and honking can startle them causing a crash.
Always think that the bicyclists coming towards you in the other lane is traveling faster then you think they are. On a down hill course a bicycle can reach speeds greater then 40 mph.
Please be tolerant of the bicyclist in front of you. The bicyclist does not want to delay you, he does not want you behind him, but wants both you and himself to be safe. As soon as it is safe for you to pass the bicyclist will move over indicating that it is safe. Ask yourself, what is so urgent that a few seconds delay will causes a major problem? Sitting at a traffic light can result in 2 or more minutes delay in your travels.
Please don't talk on the phone while driving. If the call is that important, please pull off the road to talk. Doing so also makes you safer.
Please don't use the shoulder of the highway to pass other motorists making a left turn.
Please don't pass on hills where you can't see if there is oncoming traffic due to the shape of the hill.
Please don't pass on curves where you can't see if there is oncoming traffic due to the shape of the curve.
Please don't pass within 100 ft of intersections. Not only is this unsafe, it is also illegal.
Please remember that sharing the road is a give and take process by all parties using the road.