At our Canine Training Institute we harp a lot on socialisation and leadership. For a dog to be social, or socialised, towards other dogs, humans and other animals means that:
This does not mean your dog has to become best friends with every other dog, human and animal.
The best time to socialise a dog (or teach tolerance) is actually between 8 and 16 weeks of age (the only window of opportunity you are going to get), and thereafter maintaining exposure for another year.
The best way to socialise a dog is to expose the young dog repetitively to other dogs, humans and animals on such a frequent basis that your dog accepts these as part of his life.
The importance of socialisation speaks for itself. If your un-socialised dog does not tolerate other dogs, humans or animals then you will not be able to bring your dog into the same area/space with these creatures as your dog will freak out - normally in an aggressive way. Anything that your dog does not know and he encounters this for the very first time at an adult age will scare him. Most scared dogs react aggressively towards scary things as they believe that the scary thing may hurt them. I hope the above will encourage you to socialise (expose) your young dog to other dogs, humans and animals in order for your dog to learn to tolerate them.