Dogs are predators, they are designed to hunt and kill to survive. Chickens(and especially chicks) are prey. Dogs(with a few exceptions) will naturally kill any small creatures that they can catch. Your dog can be very friendly to you, and also make a habit of killing bunnies in your yard, skunks, oppossums, turtles...and they will not think twice about killing your chicks.
Training can help your dog to learn that YOU can touch the chicks and he can not ever touch them without you giving him permission...but you should never trust your dog alone with them. To be on the safe side, I would just keep him away from your chicks at all times, AND make sure he cannot get into their coop.
My dog is a small, ornamental breed, a Japanese Chin, and she has been raised her entire life knowing that she may not touch any other animal in the house without my permission. I have trained her this way because I keep all kinds of exotic animals, from reptiles to bugs to birds, and I rescue/rehab injured ones too. So, she is always with me when I care for my animals, but I continually reinforce the idea that they are my animals, not hers, and she must respect them. She does a very good job, and is so happy to follow me through my caretaking routines every day.
So, training your dog is not mean, it is necessary around chicks. It actually makes your dog happier because he knows what he can happily do, and what he is not allowed to, so you don't have to actually punish him for making lots of mistakes. Train your dog, keep him completely away from your chicks when you cannot be right there, and you will both be happy!