Its not just cars – it can be a bike, bus, truck, or possibly anything which is moving at a speed fast enough to trigger some instinct in the dog to start running after it! It’s silly – the vehicle can squash them flat, but perhaps they don’t know that – or they don’t care and prefer to hound after it.
Well here’s the reason behind this ridiculous behavior: Dogs, bears, and other animals which are natural hunters, instinctively run after things which move away from them. Their inclination to hunt makes them want to capture any object which is ‘running away’ from them.
The exercise does them good, and perhaps they enjoy the feeling of running out the rush of adrenaline – just like we do.