The single most important thing you can do for your dog at a young age is to socialize them.
This fits nicely with the idea of a lack of socialization. The typical pet dog leads a relatively constrained life. Leashes, baby gates, crates, usually indoors, etc.
Like people, dogs yearn for security and protection all the time. A dog's desire to feel secure increases as its anxiety and terror levels rise.
This is a continuation of number 3. In some circumstances, dogs get nervous and anxious. When your annoying nieces visit the family, for instance
The wonderful thing about dogs is that they are always communicating with us. Their emotional state cannot be concealed.