Not all dogs like to be groomed. Some get scared or nervous when the time for a bath or a brushing comes. This is natural. All it takes to overcome it is positivity-based training. Gradually introduce the grooming process to your dog with treats to help him adjust to the process and develop positive associations with it.
Easing Into Grooming
Instead of trying to introduce grooming quickly, ease your dog into his comfort zone by taking it slowly. For example, instead of forcing him into the tub for a bath -- which some dogs find unpleasant -- practice taking him into the empty tub once a day. Give him a treat while he's in there. He'll learn to associate bathtub time with treats and positive encouragement. Similarly, introduce other aspects of grooming slowly. Try brushing him for just a few minutes a day, rewarding with a treat afterward, and gradually working your way up to a full brushing session. Clip only one or two nails at a time before giving him a treat and continuing. Whatever you need to do to groom your dog, taking it a little bit at a time, with plenty of encouragement along the way, will comfort his pre-grooming nerves.