In the short story 'A Rose for Emily' the main character exhibits several traits that enable her to keep up the task of staying withdrawn from the world - she probably herself wouldn't class this as loneliness as it is self-chosen, self-imposed isolation. This kind of trait can come under the more general term of 'dissociative personality disorder.' Dissociative types tend to display this trait early, often (as mentioned in the above post) brought on by early childhood or babyhood experiences. Sometimes it is the parents who have the actual problem - deliberately keeping their child from building relationships with others out of a possessive need to hold on to them. So the children never learn social skills. The other things Emily has are her memories - she clings on to the idea of Homer Baron rather than the real living person - in every last way she can, something she takes to worrying extremes! She takes refuge against the real world in them.
Sure! If you look at her personal traits throughout the story, a lot of them contribute to her ability to be alone all of those years. Her most defining trait is stubbornness. She is incredibly stubborn about her family's position in the town, and its responsibilities and duties. She refuses to pay the taxes that she owes, simply because her grandfather and father had a deal with the mayor that stated she didn't. Times changed though, and she owed taxes, but, she stubbornly refused. She was stubborn about letting people in the house after her father died, stubborn about letting go of Homer Barron, and stubborn about the townspeople's opinions of that relationship. That stubbornness probably lent itself to her isolation. If she was lonely, her being stubborn probably kept her in the house, isolated. She was too stubborn to descend to friendship with others that were below her. Her stubborn view of what she and her family should represent kept her isolated from modern times and perceptions that would have allowed her some companionship.
Another trait that kept her isolated was her naivety. Being naive means that you don't really have a mature perspective of the world, and just assume everything fits your worldview. She was naive about Homer Barron and his attentions, which gave his rejection that much more of a shock. She was naive about her father being alive and with her for her entire life. She was naive about how the townspeople viewed her and her family. That naivety kept her alone, because she wasn't able to see things for how they were.
You can't pass up her eccentricity. Growing up alone with an over-protective father made her very strange and eccentric. He trained her to be alone, practically, by keeping her from marrying happily, and from keeping her from those below them. She had eccentric views about slaves, keeping a male slave only in the house for years, eccentric in her behavior to Homer, and eccentric after her father died. She was a bit of a weird lady, and that made it possible for her to be so alone all of the time. Her eccentricity probably made it so it wasn't so weird for her.
Those are just a few traits that aided her isolation; I hope that helped. Good luck!