As mentioned in our post about the concept of Least Invasive, Minimally Aversive, punishment easily backfires, causing fear and aggression. 

Fear

• To avoid fearfulness, a puppy should meet and been held by 5 strangers a day from ages 8 weeks until 12 weeks, visit many floor surfaces, and get cued to come to dinner by banging pots and pans (per Dr. Ian Dunbar).
• If the dog is fearful because they did not get this proper socialization as a puppy, find a trainer who will teach you the "Look at That" game (invented by Leslie McDevitt). It's tricky to do, but a trainer can teach you how.

Aggression

• This behavior tends to come from fear.
• Just like when a dog is fearful, find a trainer who will teach you the "Look at That" game (invented by Leslie McDevitt). It's tricky to do, but a trainer can teach you how.