Instincts
Your dog exhibits many behaviours which are instinctual to him. In other words, he cannot control them easily because he reacts to certain stimuli which will switch on the behaviour associated with that stimuli. For instance, chasing squirrels. It is very difficult to stop your dog chasing something that moves. You will notice he will instinctively chase anything, even a fluttering leaf or a piece of paper being blown by the wind. The prey drive is very strong in many dogs, hence some are used to herd cattle and sheep. But they are conditoned not to harm just to herd.