Why do people slow down when their car is next to a truck on the highway?

I hope I am not the only one who has repeatedly endured this. You are in the left lane, the car in front of you is next to a truck (which is in the adjoining lane). Car in front of you slows down instead of speeding up as would seem the more logical thing to do. Why is this? Is there some great reasoning behind this that I am missing?

I would think that you would want to speed up to pass the truck instead of staying at the same pace as the truck. Driving right next to the truck makes your car an automatic target for things flying at it.