During his childhood, America experienced several periods of
loss and loneliness that ultimately impacted his personality. In turn, he developed
a chronic fear of being alone.

The most obvious event that comes to mind is the times where he was still under England’s care. England would leave for long periods of time
without visiting. America was terrified at the prospect of this and would beg
England not to leave.

Hints of the resulting loneliness he experienced during
these gaps in visits become obvious as young America picnics alone and achieves
some condolence by convincing himself that he’s not lonely [x]. 

While his
experiences in the Wild West may have caused him to harden and evolve from his
previous crybaby personality [x], his fear of being alone remained. I would even
argue against this piece of canon material and say that America’s still a
crybaby as an adult (other material contradicts it). 

Another event that I would say had an impressionable impact
on America is the loss that he experienced with Davie. While he may have been
too young to comprehend immortality and mortal death, America still experienced
the loss of a close friend. He had gotten attached to someone only for them to
leave [x]. 

What’s interesting is that even after America gained his
independence and pushed England out of his political affairs, America was
unable to reconcile the idea of having a personal life without England. It’s
for this reason that I believe America goes to Canada’s house in desperation,
asking that he be able to see England who had fallen ill from the conflict [x]. 

On the other hand, America also detaches himself from the
pain of the loss that he experienced from the Revolution by storing away
objects that reminded him of his childhood with England (i.e., the toy soldier, suit, and gun) [x]. 

As mentioned, America’s fear of being alone has remained in adulthood:

He’s jealous of kids who get presents from their fathers on
Christmas [x]. 

He has a notable habit of asking other nations to sleep with
him after he’s watched/ read scary horror material [x] [x]. 

He clung to Japan during a time where all other nations
weren’t favourable of him [x]. 

The last is a speculative conclusion that hasn’t been
confirmed. England ties America’s tendency to mass produce to the fact that
he possessed very little belongings when building new settlements in his
country [x]. What I believe has  merit is the argument that America turned to
stuff to help cope with his loneliness.