A dog is nervous if it is apprehensive due to anticipating an unidentified threat, situation or danger, this can be overcome and most dogs go on to have happy well socialised lives. You cannot understand why it is nervous but you can help it overcome with positive reinforcement, love and perseverance.If your dog is of a nervous disposition try not to molly coddle it, Take it out where it can see other dogs but isn't too close that they might worry it. Try taking it to training classes but if it shows signs of panic or fear, like tail very low and often between it's legs, ears down and back fidgeting, calmly leave the class, don't pet your dog or speak to it, just walk him round for a bit and then take him home.Find a walking buddy that has a calm, easy going dog that you can walk with, if there are none in your area try putting an advert in your local paper. When you do get a buddy and you are out walking, chat cheerfully to your buddy, your dog will pick up on this and slowly lose it's nervousness with the dog you are walking with. Don't let your dog off the lead yet as it may panic and run away from you and the other dog.Once your dog is happy to walk along with the other dog, see if you can find another buddy so that there are three dogs in all, do the same as you did with the first dog, slowly and calmly.If your dog is nervous about being outside, try seeking your vets advice, agoraphobia could be an issue, although this is unlikely.When your dog is relaxed and happy walking with your walking buddy's dog try the training classes again. Watch your dog closely but do not speak to it, avoid eye contact with it. If it seems happy to sit close to you and watch the other dogs without showing any outward signs of fear, tell him 'good boy' and give him a treat. if, however, he is still fearful and nervous, leave the class. Keep going back to the class and try and stay a little longer each time, only praising your dog when he shows no sign of nervousness. Also keep taking him to places where there are other dogs playing, but, always keep well away from the other dogs. If they appraoch your dog because you are too close, your dog may attack them through fear, or he will try to run away and undo any progress you may have made.When your dog actually shows signs of wanting to be in the company of your walking buddy's dog, e.g. gets excited at the thought of a walk, or when he sees the other dog, take them to a secluded area, not your back garden or the other dog's back garden this is each of their territory and they will be on guard so to speak. When you find a secluded area that is safe and the dogs cannot run into trouble, let them off. If your dog is happy in this situation keep this as a regular date, once ot twice a week.Meanwhile try the training class again. This time if your dog is fairly relaxed try joining in with the class. Watch your dog for any signs of distress. if he manages even five minutes of the actual lesson, praise him generously and leave the room calmly. Continue with his buddy walks and playtimes while also trying him in the park with the other dogs in the distance. If he looks interested but not nervous of the other dogs move a little closer to them watching your dogs body language. The moment he looks uneasy, stop, walk back a few steps and let him watch from there.Talk to the training class instructor and see if they could do one or two classes with your dog and maybe one or two other placid dogs. Most will be happy to oblige. If this happens take your dog in before the others arrive, let him sniff around for a few minutes to get the feel of the room or yard. if he is fairly calm and shows no signs of panic, praise him and give him a treat. Get him to sit while the other dogs are brought in. The owners should have been advised of the circumstances of this exercise so they won't let their dogs bound over to yours.If your dog is quite happy, walk round, talk to your dog, 'good boy', 'heel' are good words to say. No coo-ing noises. The other dogs should then fall in, in front of you doing exactly the same as you are doing until all dogs are walking. Try not to let one of the other dogs get too close behind your dog, this will make him turn and he will ignore your voice. If you manage 10-15 minutes of this then you are well on the way to conquering your dogs nervousness, so don't give up. keep going to the training classes, walking with your buddy until your dog joins in without a care or worry in the world.Patience and perseverance will win. If and when you feel your dog is ready to join in with the rest of the class, let the instructor know and see if you can be integrated slowly.