People are always surprised when later in life, their friendly and socialized puppy becomes shy, standoffish, wary, fearful, reactive and aggressive towards people or other dogs, even though developing fear of the unfamiliar later in life is a normal and adaptive development process.
These problems can be easily prevented with lots of early socialization and ongoing classical conditioning. For older dogs that are past the Critical Period of Socialization, the treatment protocols are virtually identical to prevention protocols (classical and operant conditioning, progressive desensitization and socialization), except that they are more complicated, take much longer and are sometimes dangerous.
Recently though, new techniques have been developed to accelerate and maximize classical conditioning, progressive desensitization and ultimately, re-socialization. Differential classical conditioning (DCC) enables trainers to operantly condition at the same time as classically condition the dog. Set-ups accelerate classical conditioning and using kibble and tug toys (fairly low-value primary reinforcers) empowered as the highest-value mega-secondary reinforcers maximizes classical conditioning.