Any dog not socialized properly is unfriendly. Terriers seem to graviitate in that direction, they need lots of socializing, they have a high prey drive to do their jobs. Small toy dogs that don't have a job to do, except look cute and keep our laps warm, tend to turn out this way due to the fact that the owners make them this way. They carry them around like babies and they don't get to be a dog, on the ground, dealing with their environment from their own level.
Plus misinformed vets and breeders scare the day lights out of us and tell us they will be stricken by diseases and such if taken around anything until they get their full set of shots. Here in the USA that is 4-5 months old. The primary socializing age is 4 weeks to 16 weeks, after 16 weeks, it is much harder to socialize them, not impossible but harder, spay/neuter helps, and a good breeding of even tempered dog and bitch to produce even tempered pups helps also. It's nature vs nurture with dogs. I have known friendly westies and aggressive westies, the aggress ones came from bad breeding or not socialized early or both. Good ones were in a puppy class and taken all kinds of places to get use to things, people and other animals, cats included. Sometimes nature wins and you get the odd one out that they tried to do everything for it, but still is aggressive. Training is key when you get a dog.